WASHINGTON >> The race for control of Congress is as close as ever, with barely two dozen House seats and a handful in the Senate likely to determine the majority this November and whether a single party sweeps to power with the White House.

Lawmakers are returning to Washington for a three-week legislative sprint, away from the campaign trail where races have become “trench warfare” and a seat-by-seat slog. Many of the highest-profile races are being waged in Montana, New York, California and beyond, far from the presidential battleground states contested by Republican Donald Trump and Democrat Kamala Harris.

“Buckle up,” said Jack Pandol, communications director for the National Republican Congressional Committee.

Upended by the summer shake-up that replaced President Joe Biden with Harris atop the Democratic ticket, the down-ballot campaigns enter this fall stretch at a virtual toss-up, a high-wire uncertainty where every single seat won or lost could make the difference.

What’s changed is not so much the fundamentals of the individual races, but which side has the energy and enthusiasm to make sure their voters actually show up and cast their ballots, strategists said.

Money, volunteers and voter enthusiasm are flowing to the Democratic campaigns since Harris replaced Biden. That’s challenging Republicans who entered the election cycle favored for gains and buoyed by Trump’s comeback bid, despite the criminal charges hanging over his potential return to the White House.

Trump and Republicans are working feverishly to regain the momentum they enjoyed from the GOP convention in Milwaukee and from the Supreme Court decision giving former presidents broad immunity from prosecution, including for some acts related to his effort to overturn the 2020 election and for the Jan. 6, 2021, attack on the Capitol.

Trump’s campaign staff held a private conference call Friday with House Republicans, assuring them that the movement is shifting to Trump as they game out strategies ahead, according to another Republican who spoke on condition of anonymity to discuss the closed conversation.

“There’s a lot of handwringing going on and a lot of anxiousness about where this election is headed,” said Montana Sen. Steve Daines, a Trump ally who heads the National Republican Senatorial Committee, the campaign arm.

Speaking at the Republican Jewish Coalition in Las Vegas,

Daines played up GOP Senate candidates as warriors and predicted that enthusiasm from rural voters who will “crawl over broken glass” to vote for Trump will help Republicans such as Sam Brown, who is challenging Sen. Jacky Rosen, D-Nev.

Still, the fundraising gap Republicans now find themselves facing with Democrats is a problem, say GOP strategists.

“We have a lot of work to do,” Daines said.

Long gone are the days of supermajorities in the House and Senate, replaced by a new era of razor-thin margins that leave little margin for errors in political campaigns, or actual governing.

Senate heads toward 50-50 split

Democrats are almost certain to see their narrow majority slip to at least a 50-50 split with Republicans with the retirement of independent Sen. Joe Manchin of West Virginia. His departure is making way for Republican Gov. Jim Justice to handily win that seat.

Trump is wildly popular in Montana.

Senate Republicans had the advantage this this cycle, with few incumbents to protect, allowing them to challenge Democrats with handpicked, often wealthy recruits in Pennsylvania, Ohio and Wisconsin. Democrats have only more recently gone on offense in long-shot races against Republican Sens. Ted Cruz of Texas and Rick Scott of Florida.

But incumbents often bring longevity and name recognition to the race, making them tough to topple, as is the case in Pennsylvania, where Democrat Sen. Bob Casey is being challenged by Republican Dave McCormick, and in Ohio, where Sen. Sherrod Brown is running a playful cookie-eating television ad as he faces Republican Bernie Moreno.

For the open seat in Democratic-heavy Maryland, the state’s popular former Republican governor, Larry Hogan, who was courted to run by Senate Republican leader Mitch McConnell, is in a tough matchup against county executive Angela Alsobrooks. She would make history as one of the few Black women elected to the Senate.

Senate Majority Leader Chuck Schumer, D-N.Y., has predicted Democrats will keep their majority.

“Democrats have never been in a stronger position to defend our Senate majority,” said Michigan Sen. Gary Peters, who leads the Democratic Senatorial Campaign Committee.

House majority a toss-up

In the House, Speaker Mike Johnson, R-La., has crisscrossed the country this summer in some 20 states as what he calls an “ambassador of hope” in his party’s quest to save its razor-thin majority.