Hues of red and blue color the dawn at the Capitol in Washington on Monday. Control of Congress and of President Biden’s agenda on Capitol Hill are at stake this Election Day. J. Scott Applewhite/Associated Press

WASHINGTON — Eager to claw back power in Congress, energized Republicans waged a grinding push late Tuesday in hard-fought races as they struggled to break the Democrats’ one-party hold on Washington and upend President Joe Biden’s once-lofty agenda.

While polls began closing from the East Coast toward the West, the Democrats’ fragile grasp on power was at risk. With the narrowly held House and an evenly split Senate, the party faced a new generation of Republican candidates – among them political newcomers, including deniers of the 2020 election and some extremists inspired by Donald Trump handily winning some seats. But races were tight.

The Republicans could bring a new intensity to Capitol Hill with promises to end Biden’s most ambitious plans and to launch investigations and closer oversight – even, potentially, impeachment of the president.

Tuesday saw the first major national elections since the Jan. 6, 2021, attack on the Capitol, and emotions were raw. The recent violent assault on Speaker Nancy Pelosi’s husband has stunned many, and federal law enforcement warned of heightened threats nationwide. Biden’s party worked to hold on by the most tenuous of margins.

“We intend to win,” Pelosi told the PBS “NewsHour,” insisting that Democrats have “far superior candidates” and voters will turn out to support them.

“So I think you’ll be surprised this evening,” Pelosi said.

All 435 seats in the House and one-third of the Senate were being decided. If Republican newcomers help the party seize control of the House, and possibly the Senate, the outcome will pose new challenges for Congress’ ability to govern — especially if margins are tight.

Early on, four new senators were elected: Democrat Peter Welch of Vermont and Republicans Katie Britt of Alabama, Markwayne Mullin of Oklahoma and Eric Schmitt in Missouri. They will replace retiring senators.

Senate Majority Leader Chuck Schumer, a Democrat, was reelected in New York. Republican Sens. Rand Paul in Kentucky and Marco Rubio in Florida prevailed over their Democratic opponents. In Colorado, Democratic Sen. Michael Bennet also won reelection.

In the House, several new Republicans were elected in redrawn Florida districts. Joining them will be 25-year-old Democrat Maxwell Frost – the first member of Generation Z to win a seat in Congress.

Far-right Rep. Marjorie Taylor Greene, a top Trump ally, won reelection in Georgia.

But Democrats held on in other areas. Virginia Rep. Abigail Spanberger defeated Trump-backed Republican Yesli Vega in a closely watched district Republicans wanted to flip.

Divided government has historically offered the possibility of bipartisan deal-making, but Republican candidates campaigned instead on a platform to stop Democrats.

“I do think that this will end up being a period of government that is defined by conflict,” said Brendan Buck, a former top aide to the past two Republican speakers of the House.

Without a unified agenda of their own, Republicans ran on threats of confrontations that could spark crises as they promised to cut federal spending, refuse to raise the nation’s debt limit and balk at supporting Ukraine in the war with Russia. It all pointed to potential gridlock ahead.

“They’re going to make very clear that there’s a new sheriff in town,” Buck said.

House Republican leader Kevin McCarthy, who is in line to seize the speaker’s gavel from Pelosi next year if Democrats lose power, has recruited the most racially diverse class of House Republican candidates, with more women than ever. But it also has a new cadre of Trump loyalists, including election skeptics and deniers, some of whom were around the Capitol on Jan. 6.

Trump has endorsed more than 330 candidates nationwide this election cycle, including more than 200 for the final House and Senate ballots, though they were not always the first choices of McCarthy and Senate Republican leader Mitch McConnell. In an interview, the former president said he backed McCarthy for speaker, and he derided his old foe McConnell as a “lousy leader,” according to Fox News.

In a sign of the nation’s toxic political climate, Pelosi canceled most public appearances in the final week of campaigning after an intruder broke into her family’s San Francisco home in the middle of the night, demanding to know “Where is Nancy?” and bludgeoning her 82-year-old husband in the head with a hammer. Authorities have said the Oct. 28 attack specifically targeted the speaker’s home.

The election unfolded amid deep discontent. A majority of Americans – about 7 in 10 – disapprove of the way Congress is handling its job, according to AP VoteCast, an expansive survey of over 90,000 voters nationally. About 4 in 10 strongly disapprove.

In the fight for the House, Virginia’s marquee race between Democratic Rep. Elaine Luria and Republican challenge Jen Kiggans, both Navy veterans, provided a snapshot. The two-term Democrat Luria, first elected in the 2018 backlash to Trump, rose as part of the committee investigating the Jan. 6 Capitol riot but is now at risk of defeat.

The Senate battleground was focused on four deeply contested states where razor-thin margins could determine outcomes – in Georgia, Arizona and Nevada, where Democratic incumbents are trying to hold on. In Pennsylvania, the race between Democrat John Fetterman and Republican Mehmet Oz for an open seat was considered key to party control.

One other Senate contest that will be closely watched is in New Hampshire, where Trump-styled Republican Don Bolduc is trying to oust Democratic Sen. Maggie Hassan in a race that could signal the former president’s viability with voters two years after he left office.

Vote counting could extend beyond Election Day in many states, and Georgia in particular could head to a Dec. 6 runoff if no candidate reaches the majority. Both parties have already filed legal challenges in some cases foreshadowing the court fights that may delay final results.

Republicans need a net gain of five seats in the House to achieve the 218-seat majority and a net gain of one to seize control of the Senate. The 50-50 Senate is now in Democratic hands because Vice President Kamala Harris can cast a tie-breaking vote, in what has been one of the longest stretches of a split Senate in modern times.

Inflation, abortion, crime and the future of democracy have all been at the forefront of campaigns as candidates strive to reach voters.

Democrats gained momentum over the abortion issue after the Supreme Court overturned the Roe v. Wade decision this summer, and they have been warning voters about MAGA conservatives, short for Trump’s “Make America Great Again” slogan.

But Republicans have focused voter attention on closer-to-home issues — inflation’s high prices and crime — as they tap into unease over the country’s direction.

Senate Republican leader McConnell openly griped about “candidate quality” potentially costing his party victories, as Trump championed his preferred candidates to create a potentially untested class of newcomers.

House Democrats faced their own recruitment problems, a situation worsened by the slew of Democratic retirements as longtime lawmakers headed for the exits, some giving up their committee gavels rather than accepting a career in the minority party.

In one dramatic example of the difficult political environment for Democrats, the party’s House campaign chairman Rep. Sean Patrick Maloney was fighting for political survival against Republican state legislator Mike Lawler in New York’s Hudson Valley. He would be the first Democratic campaign chief to suffer defeat in two decades.