WASHINGTON—Republicans are playing defense on an expanding electoral map, spending hundreds of millions of dollars in the final weeks of the midterms to prevent a Democratic takeover of Congress as campaigns nationwide reach a fever pitch.

With less than three weeks until the midterms, deep-pocketed GOP groups are trying to push struggling candidates over the finish line, while President Trump and Vice President JD Vance are crisscrossing the country to shore up Republican support in red areas that seemed virtually unassailable just months ago.

Democrats, working to capitalize on Trump’s weak approval ratings and economic concerns in the wake of the war in Iran, have expanded their target map into more Republican-leaning territory. That has forced Republicans to pour in massive sums to defend traditionally safe red seats, while also attempting to pick off some seats in purple states such as Michigan and New Hampshire to help stanch potential losses on their home turf.

In the waning days of campaign season, Republicans hope money and structural advantages in the electoral map will buoy them—and that polls are wrong. But anxiety is running high.

“The House is gone. It’s been long gone” said Doug Heye, a Republican strategist. He said that Democrats could potentially flip 30 seats in November given Trump’s low favorability, adding that he didn’t see the president’s rallies helping. Meanwhile, Democrats are feeling optimistic about their prospects.

“It just feels like America is the battleground state,” said John Anzalone, a Democratic pollster who helps conduct The Wall Street Journal’s surveys. “Everything is up for grabs.”The nonpartisan Cook Political Report said recently that the most likely outcome is Democrats flipping a net of nine to 19 House seats. Republicans currently have a 219 to 214 majority, with two vacancies. Cook ranks seven Senate races as tossups—six of which are currently held by Republicans. The GOP has a 53-47 majority in that chamber.

Voters are already casting their ballots in several states across the country, including Arizona, Iowa and Kansas. In Georgia, voters set a first-day early voting record on Tuesday, with more than 188,000 ballots cast in person, beating the record set in the 2022 midterms, the secretary of state’s office said. Georgia, a battleground state, has competitive Senate and governor races this year.

Trump, who has continued to urge voters to turn out as if he was on the ballot, rallied Wednesday in Arizona, where there are competitive House and governor races, after stops earlier in the week in Ohio and Alaska to try to shore up the campaigns of GOP Sens. Jon Husted and Dan Sullivan, respectively.

Trump is set to campaign next week in North Carolina, where the effects of the Trump-initiated redistricting are making at least one of their incumbents more vulnerable. Republican Rep. Greg Murphy of North Carolina is facing a more competitive race than expected after his district lost Republican voters, following map changes that were meant to boost GOP fortunes.

On Tuesday night, Senate candidates from marquee races in Georgia, Maine and Iowa all faced off in debates. Democratic Sen. Jon Ossoff of Georgia and Republican Sen. Susan Collins of Maine are both incumbents fighting to keep their seats—and Iowa is an open race. Democrats in all three contests attacked Republicans over the Iran war and high prices.

Control of Congress during the final two years of Trump’s second term could have major implications for his agenda and shape how much momentum either party has looking to the 2028 presidential election.

Republicans are flexing their money advantage over Democrats and throwing hundreds of millions of dollars into races across the country. Still, polls haven’t shown significant shifts in high-profile contests since more money started pumping in, with key Senate races largely unchanged.

“What we need to see is those ad dollars transferring into voter enthusiasm,” said Mark Bednar, a Republican strategist, pointing out a consistent voter enthusiasm gap between Democratic and Republican voters. “Republican voters need to get excited in this final stretch.”Republicans entered this election cycle with a significant advantage on money. GOP advertisers have outspent Democrats by almost $298 million across eight competitive Senate races—Ohio, Texas, Maine, Michigan, Iowa, Alaska, New Hampshire and Kansas, according to a new report from ad-tracking firm AdImpact.

The Senate race in Ohio between Husted and former Sen. Sherrod Brown is the most expensive Senate general election this cycle in terms of ad spending, with more than $422 million spent, the report found.

The matchup between Democratic state Rep. James Talarico and Republican state Attorney General Ken Paxton in Texas came in second—and is one where Republicans are dramatically outspending Democrats. Outside Republican groups have poured in over $209 million in the general election, the report found. Much of that money is coming from Texas PAC, a political-action committee run by the Senate Leadership Fund, the big-donor group aligned with Senate Republicans.

Last week, the Senate Leadership Fund shifted more money toward the Texas Senate race. Rather than working to rehabilitate Paxton’s image, as he has faced a number of legal and ethical scandals, Republican spending has focused on trying to tear down Talarico, according to a Republican working on Senate races.

Despite the aggressive Republican spending in both Senate and House races, Democrats have sought to expand their map of target races.

In a donor memo from the Democratic Congressional Campaign Committee on Friday, the House Democratic campaign arm’s executive director, Julie Merz, made the pitch that if the group can raise an additional $17 million by Oct. 22, the party can play in more races.

“We need more resources to close the sale in these winnable seats,” wrote Merz.

The letter cited races such as Florida’s 27th Congressional District, where GOP Rep. Maria Elvira Salazar is fighting off Democratic challenger Eliott Rodriguez as well as South Carolina’s 1st District, where the House Republican-aligned Congressional Leadership Fund is now spending.

Write to Sabrina Rodriguez at sabrina.rodriguez@wsj.com and Olivia Beavers at Olivia. Beavers@wsj.com