Updated Nov. 9, 2016 4:32 a.m. ET
Republican victories in the House and Senate Tuesday ensure that GOP president-elect Donald Trump will begin his term in the White House with his party in full control on Capitol Hill, though Senate Democrats even in the minority will retain some power in a chamber designed to forge compromise.
The GOP’s alliance will be uneasy from the start, however, given congressional Republican leaders’ deep differences with Mr. Trump, whose policy stances and contentious comments they criticized during the long and turbulent presidential campaign.
Both Senate Majority Leader Mitch McConnell (R., Ky.) and House Speaker Paul Ryan (R., Wis.) congratulated Mr. Trump on his victory early Wednesday morning.
“We are eager to work hand-in-hand with the new administration to advance an agenda to improve the lives of the American people,” Mr. Ryan said in prepared remarks.
Republicans in both the Senate and the House will be sworn in January with narrower margins than they had this year, even though Republican candidates in both chambers performed better than expected. Across the map, Republican incumbents in tough races were able to hang on against some carefully chosen Democratic challengers.
As of about 3 a.m. Wednesday, Republicans had lost only Illinois, where Democratic Rep. Tammy Duckworth defeated incumbent Sen. Mark Kirk. The New Hampshire race, where Gov. Maggie Hassan posed a stiff challenge to Republican incumbent Kelly Ayotte, remained too close to call well into the morning.
Two former senators, Evan Bayh in Indiana and Russ Feingold in Wisconsin, weren’t able to leverage their statewide name recognition to win seats in those states. Rep. Todd Young won in Indiana and incumbent Sen. Ron Johnson in Wisconsin was re-elected with 51% of the vote to Mr. Feingold’s 46%, with 99% of precincts counted.
After months of fretting that Mr. Trump would weigh down incumbent Senate Republicans, his strength instead buoyed GOP lawmakers in battleground states. With no wave of support materializing for Democratic nominee Hillary Clinton to bolster down-ballot candidates, Republicans swept through swing states, including many that President Barack Obama had won four years earlier.
With the victory of Sen. Johnson of Wisconsin, whose re-election had been dismissed as a long shot until the election’s final weeks, Democrats’ chances of seizing the Senate majority dwindled to a remote possibility. GOP Sen. Pat Toomey’s win in Pennsylvania over Democrat Katie McGinty early Wednesday morning made it impossible.
Republicans will hold at least 52 seats in the Senate next year, assuming that Republican John Kennedy wins the Louisiana runoff in December, as expected. Democrats needed to win a net of five seats to recapture the Senate majority they lost in 2014.
Republicans’ control of Capitol Hill will have dramatic legislative repercussions with Mr. Trump in the White House, positioning them to carry through on their pledges to repeal the 2010 health-care law and dismantle the Dodd-Frank financial overhaul.
Full GOP control of both chambers of Congress and the White House will leave Republicans with the political responsibility of any legislation they enact, a burden they have avoided while Mr. Obama was in office. The last time the Republicans held the White House and both chambers of Congress was from 2003 through 2007, when George W. Bush was president.
The Senate also will be under pressure to confirm Mr. Trump’s nominees to the Supreme Court, which has had a vacancy since Justice Antonin Scalia died in February. Senate Democrats, who have lambasted Republicans all year for refusing to consider Merrick Garland, Mr. Obama’s nominee to the high court, will have to decide whether to try to stymie Mr. Trump’s picks. That could leave Republicans considering whether to change the chamber’s rules to allow Supreme Court nominees to be confirmed with just a simple majority, rather than the 60 votes currently required for high court nominees.
Still, Republicans came short of winning the 60 seats most bills need to clear the Senate, forcing Democrats and Republicans to work together next year against the backdrop of a polarizing president and intraparty tensions.
Congressional Republicans, led by Messrs. Ryan and McConnell, will have to navigate an uncertain relationship with Mr. Trump, an unpredictable and untested politician.
GOP leaders repeatedly broke with Mr. Trump over the past several months. Mr. Ryan said in early October that he would no longer defend the GOP nominee after a 2005 video surfaced showing him talking in crude sexual terms about grabbing women without their consent, though Mr. Ryan later said he voted for Mr. Trump.
Messrs. McConnell and Ryan criticized Mr. Trump this year for his proposals to ban Muslims from entering the U.S., his praise for Russian President Vladimir Putin and his comments about women, among others. And the GOP congressional leaders have pushed to overhaul federal safety-net programs that Mr. Trump has expressed no interest in slimming down.
Republicans began the election cycle as underdogs, forced to defend 24 seats, compared with just 10 for Democrats. But Mr. Trump never became the drag Republicans had feared would cripple their chances in swing states.
GOP gains accumulated steadily Tuesday night and into Wednesday morning, as incumbent Republicans stacked up wins in states that tipped in favor of Mr. Trump.
Republican Sen. Rob Portman of Ohio easily beat back former Democratic Gov. Ted Strickland, while in Florida, Sen. Marco Rubio won re-election after losing the GOP presidential primary earlier this year. Three races seen as tossups were won by Republicans, as GOP Sen. Richard Burr beat Democrat Deborah Ross in North Carolina, GOP Sen. Roy Blunt defeated Democrat Jason Kander in Missouri and Mr. Young won Indiana’s open Senate seat to replace retiring GOP Sen. Dan Coats.
Democrats held the Senate seat in Nevada being vacated by retiring Minority Leader Harry Reid, with Democrat Catherine Cortez Masto beating GOP Rep. Joe Heck.
GOP Sen. John McCain of Arizona, once worried that Mr. Trump could cost him re-election, easily defeated Democratic Rep. Ann Kirkpatrick to win a sixth Senate term.
Earlier in the week, Democrats said they would need to work with Republicans in a chamber that runs on bipartisan cooperation.
“There’s almost a moral imperative to get things done and break through the gridlock,” Sen. Charles Schumer of New York, who is expected to replace Mr. Reid as minority leader, said in an interview this week. “If gridlock persists, then the sourness, the anger, the disillusionment of the American people will grow.”
—Arian Campo-Flores contributed to this article.
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Tags: Charles Schumer, Donald Trump, John McCain, Kelly Ayotte, Marco Rubio, Mitch McConnell, Paul Ryan, Republican victories in the House and Senate, Republicans Keep Control of Senate, Rob Portman, Ron Johnson, Roy Blunt, Senate Democrats, supreme court, Tammy Duckworth, trump wins