LOS LUNAS, N.M. — The tumultuous 2018 midterm campaign, shaped by conflicts over race and identity and punctuated by tragedy, barreled through its final weekend as voters prepared to deliver a verdict on the first half of President Trump’s term, with Republicans bracing for losses in the House and state capitals but hopeful they would prevail in Senate races in areas where Mr. Trump is popular.
The president was set to storm across two states Saturday, two Sunday and three Monday in an effort to pick off Senate seats in Indiana, Florida and a handful of other battlegrounds where Republicans hope to add to their one-seat majority in the chamber. Democrats and liberal activists, galvanized by opposition to Mr. Trump, gathered Saturday to knock on doors and make turnout calls from Pennsylvania to Illinois to Washington to try to erase the G.O.P.’s 23-seat House majority.
The run-up to the election, widely seen as a referendum on Mr. Trump’s divisive persona and hard-line policy agenda, has revealed deep strains in the president’s political coalition and left him confined to campaign in a narrow band of conservative communities. Republicans’ intermittent focus on favorable economic news, such as the Friday report showing strong job growth, has been overwhelmed by Mr. Trump’s message of racially incendiary nationalism.
While Mr. Trump retains a strong grip on many red states and working-class white voters, his jeremiads against immigrants and penchant for ridicule have proved destabilizing, with the party losing more affluent whites and moderates in metropolitan areas key to control of the House.
Republicans have grown increasingly pessimistic in recent days about holding the House, as polls show a number of incumbents lagging well below 50 percent and some facing unexpectedly close races in conservative-leaning districts.
In several diverse Sun Belt states where Republicans had shown resilience, such as Texas, Florida and Arizona, their candidates have seen their numbers dip in polling as Mr. Trump has given up the unifying role that American presidents have traditionally tried to play.
Democrats are also in contention to retain or capture governorships in rust belt states like Pennsylvania, Michigan and Wisconsin that were pivotal to Mr. Trump’s victory and fertile ground for Republicans for much of the last decade.
Despite these worrisome signs, some Republican leaders saw reason for measured optimism. While Mr. Trump said Friday that Republicans losing the House “could happen,” Representative Steve Stivers of Ohio, who leads the G.O.P. House campaign committee, has continued to predict that his party will narrowly hold its majority. Republican strategists have argued that about two dozen races are within the margin of error in polling; should right-of-center voters swing back to them on Election Day, they say, Democrats could fall short of winning enough seats to take control of the House.
Republican officials were more confident about their prospects in the Senate, where they had an opportunity to enlarge their majority in an otherwise difficult year. Nearly all of the most important Senate races are being fought on solidly conservative terrain, including North Dakota, Missouri and Indiana, where Democratic incumbents are in close contests for re-election. Mr. Trump won all three states by landslide margins in 2016.
There was an unmistakable dissonance between the relative health of the economy and the dark mood of a country as voters prepared to go to the polls just days after a wave of attempted mail bombings and a massacre at a Pittsburgh synagogue that left 11 dead.
“The nation is in political turmoil,” said Representative Carlos Curbelo, a Florida Republican facing a difficult re-election in part because of Mr. Trump’s unpopularity. “The economy is roaring but the mood is so sour. It’s a very sad time in this country.”
The mood that has imperiled lawmakers like Mr. Curbelo has buoyed Democrats across the country. A class of first-time candidates has been lifted by an enormous surge of activism and political energy on the left, as a loose array of constituencies offended by Mr. Trump — including women, young people and voters of color — has mobilized with a force unseen in recent midterm elections.
Early voting across the country reflected the intensity of the election: More than 28 million people had already cast ballots by the end of Friday, about 10 million more than at a comparable point in the 2014 midterm elections, according to the Democratic data firm Catalist.
The Battle for Congress Is Close. Here’s the State of the Race.
The math currently favors the Democrats in the House and the Republicans in the Senate.
If Mr. Trump has animated a powerful national campaign against him, Democratic candidates have largely avoided engaging the president personally in the closing days of the election, instead hewing close to a few favored issues like health care.
At a Saturday morning rally, Representative Ben Ray Luján of New Mexico, the head of the Democrats’ campaign committee in the House, drummed home the party’s ethos of ignoring Mr. Trump while riding the backlash against him.
“We don’t really have to even talk about this president — he’s going to do all the talking about himself, for himself,” Mr. Luján said, addressing volunteers in Los Lunas, where Democrats are making a push to pick up an open House seat. “I want you to concentrate on families here in New Mexico.”
But Senator Martin Henrich, appearing beside Mr. Luján and Xochitl Torres Small, a water-use lawyer who is the Democratic nominee for Congress, cast the election in dire terms familiar to anxious Democrats across the country. “This is a battle for who we are as a nation,” said Mr. Heinrich, who is expected to win re-election easily on Tuesday.
That mind-set on the left has given Democrats an upper hand in campaign fund-raising. Political spending in the election is expected to exceed $5 billion, making it the most costly midterm contest in history, according to a report by the Center for Responsive Politics. The report found that Democratic candidates for the House had raised more money than their Republican competitors, by a margin of more than $300 million.
But many Senate Democrats have also decisively outraised their contenders, a sobering reminder to Republican officials about the rise of small-dollar and billionaire contributors on the left.
“If alarm bells aren’t ringing across the Republican landscape as a result of the dollars Democrats have raised and the mechanism they raised them with, then we don’t deserve the majority,” said Senator Cory Gardner of Colorado, who oversees the Senate Republican campaign arm.
Mr. Gardner warned that the Democrats’ newfound fund-raising prowess could buffet his party even more in 2020, when a less-inviting list of seats is up for election — including his own.
“We may be able to survive with this map in 2018, but we cannot survive that map in 2020,” he said.
It is the House, though, where Republicans face greater peril.
Most critical to determining control of the chamber are likely to be prosperous, culturally dynamic suburbs — around cities like New York, Philadelphia, Detroit, Miami, Chicago and Los Angeles — where Republicans are defending several dozen districts packed with voters in open revolt against Mr. Trump. Democrats have won over many swing voters in these areas with a message focused on Republican health care and tax policies that are even less popular than the president himself.
“I don’t think you can find a race in the country where health care hasn’t been a dominant issue,” the Democratic strategist Jesse Ferguson said.
The fate of Republican lawmakers in the East Coast suburbs could offer an early harbinger on election night of whether the party can maintain even a tenuous grip on the House. Many of those communities could also tip powerful governorships into Democratic hands for the first time in a decade.
Former Gov. Ted Strickland of Ohio, the last Democrat to lead that state, said the election had effectively become a referendum on Mr. Trump, leaving Democrats “confident about the House and a little concerned about the Senate.”
“He’s on the ballot, regardless of whether his name is there or not,” Mr. Strickland said of the president. Mr. Trump added that the shift in the political climate between 2016 and 2018 was like “the difference between heaven and hell.”
Mr. Trump has appeared to turn his attention in the last few days away from the effort to keep control of the House, and toward shoring up Republicans in coveted Senate races. He has focused predominantly on electrifying the right rather than soothing some of the swing voters who backed him over Hillary Clinton two years ago.
In the final weeks of campaigning, Mr. Trump has delivered slashing attacks on immigration, railing against birthright citizenship, linking immigration without evidence to violent crime and amplifying debunked conspiracy theories about a migrant caravan in Latin America. In a possible portent of how he might react to electoral defeat, Mr. Trump lashed out at House Speaker Paul D. Ryan on Twitter after Mr. Ryan criticized his dubious proposal to void the constitutional guarantee of citizenship to anyone born on American soil.
Mr. Trump’s approach may resonate in several of the states with the closest Senate races, though it has the potential to backfire in several diverse states where Republican-held seats are at risk, including Nevada, Arizona and Texas.
“It turns off independent voters,” said Senator Chris Van Hollen, a Maryland Democrat and head of the Democratic Senate campaign arm, arguing that such states offered his party “a narrow path” to a majority.
Christine Matthews, a Republican pollster, said the Democratic message, focused on health care, was “more relevant” to most voters than what Mr. Trump was offering them in his final argument. Likening the election to a tug of war, Ms. Matthews said the president was trying to energize his predominantly white and male base even as moderate women recoil from him.
“On one end, you’ve got white college-educated women pulling hard, pulling back from what we’re seeing,” Ms. Matthews said. “On the other side of the rope, you’ve got non-college-educated men pulling hard in the other direction.”
Even before his lunge toward culture-war rhetoric, Mr. Trump and his party appeared to be receiving scant political advantage from the state of the economy, which has grown steadily during his presidency and pushed the unemployment rate below 4 percent. A dip in the stock market over the final weeks of the campaign appeared to frustrate Mr. Trump, who suggested on Twitter — without evidence — that it reflected investors’ concerns about a Democratic takeover.
At no point this fall has a majority of voters approved of Mr. Trump, and while some surveys have shown improvement in his standing recently, the Gallup poll found at the end of October that just 2 in 5 Americans rated his performance favorably.
If many of the most closely watched elections are at the federal level, governors’ races around the country may be the most consequential elections, long term, for both parties. Democrats are hoping to elect a history-making set of candidates, including Stacey Abrams in Georgia and Andrew Gillum in Florida, who would be the first African-Americans to lead their states. And Republicans are struggling to defend their dominance across Midwestern state governments, from Michigan and Ohio to Wisconsin and Iowa.
Should Democrats capture several of those governorships, it could help them redraw congressional district lines to their advantage after 2020. And Republican losses at the state level could complicate Mr. Trump’s path to re-election, strengthening the opposition party in crucial battlegrounds where he failed to consolidate support after the 2016 election.
But it was the bifurcated maps this year between House and Senate that made 2018 so unusual, with Republicans from different regions sounding as if they hailed from two opposing parties when discussing Mr. Trump.
“If we pick up seats, he’s going to be the only reason why,” Mr. Gardner said, crediting Mr. Trump’s aggressive campaigning in red-state Senate races.
Yet Mr. Curbelo, who like many of the most endangered House Republicans represents a metropolitan area, portrayed the president as a mildly irritating but distant bystander.
“Republicans are running their own races in each of their districts while the president is talking about topics important to him,” he said with a wry chuckle.