Rep. Tammy Duckworth unseated incumbent Republican Sen. Mark Kirk in Illinois on Tuesday night, giving Democrats an early pickup as they seek to wrest control of the upper chamber from the GOP.
Control of the chamber remained on a knife’s edge as results trickled in late into the night, but Republicans were projected to hold on to seats in Ohio, Florida and Indiana that Democrats once thought they had a chance of winning. But plenty of other targets remained on the board as Senate Republicans struggled to defend their 54-46 majority.
The U.S. House of Representatives, meanwhile, was projected to remain in GOP hands, handing Republicans at least one check on a would-be Democratic president and Senate as both sides appeared on track to trade key House wins. The results fell well short of hopes earlier this fall of a Democratic “wave election” that would have given the party total control of Capitol Hill.
Sen. Marco Rubio, a Florida Republican who won his first re-election bid in a closely watched, expensive race, tried to reassure uneasy voters during his election night victory speech.
“America’s going to be OK. We will turn this country around,” Mr. Rubio said.
“I know you feel betrayed, and you have a right to,” he said. “But we must channel that anger and frustration into something positive.”
Ms. Duckworth’s win, meanwhile, which several networks called relatively soon after polls closed in the state, gave Democrats a must-have seat as the party also eyed other pickup chances in Pennsylvania, New Hampshire, Wisconsin and Missouri.
In Ohio Sen. Rob Portman easily won a second term in his race against Democrat Ted Strickland. Networks called the race in Ohio soon after polls in the state closed at 7:30 p.m. EST.
“Regardless of which party controls the Senate, I’m going to treat my Senate colleagues — new or old, Democrat or Republican — on the assumption that they care about our country as much as I do,” Mr. Portman said in excerpts of his victory speech.
In Florida networks called the race between Mr. Rubio and Democratic Rep. Patrick Murphy shortly after the final polls closed at 8 p.m. EST. After flaming out in the Republican presidential primary, Mr. Rubio ended up reversing course and decided to seek re-election.
In Indiana Republican Rep. Todd C. Young knocked off former Democratic Sen. Evan Bayh to fill the seat of retiring Republican Sen. Daniel Coats. Mr. Bayh at one time enjoyed a huge lead in the polls. Networks called that race about 90 minutes after final polls closed.
The Ohio and Florida Senate contests had been eyed by Democrats as prime potential pickups and took place in states where both Mr. Trump and Hillary Clinton went all out at the presidential level. And Democrats had been bullish on Mr. Bayh, a former senator who announced his retirement in 2010, after he jumped into the race over the summer.
Those contests were bright spots for Senate Republicans and GOP candidates, but they had all been forced to grapple with how closely they wanted to tie themselves to Mr. Trump.
Mr. Kirk had rescinded his Trump endorsement in June after the GOP presidential nominee implied an Indiana-born judge overseeing a civil case against him was biased because of his Mexican heritage. Mr. Kirk said he wrote in former CIA Director David Petraeus for president on Tuesday.
Meanwhile, Sen. Kelly Ayotte, locked in a tight re-election fight in New Hampshire against Democratic Gov. Maggie Hassan, said during one debate that she would describe Mr. Trump as a “role model” before quickly reversing course afterward.
“Math and history were against Democrats going into the election, but Donald Trump made the Republican nominees absolutely radioactive,” said Jim Manley, a Democratic strategist and former top staffer for Senate Minority Leader Harry Reid.
Mr. Reid worked overtime to get Mrs. Clinton across the finish line in his home state of Nevada, as well as to see his would-be replacement, Catherine Cortez Masto, succeed him in her close race against GOP Rep. Joseph J. Heck. The race marked one of Republicans’ only realistic chances to pick up a Senate seat now held by Democrats, in an election cycle in which the GOP was defending twice as many seats as their opponents.
Ms. Cortez Masto benefited from superior Democratic organization efforts and Mr. Reid’s relationships across the state, while the state Republican Party was less organized and Mr. Trump hadn’t put many resources into ground game efforts, said Dan Lee, a political science professor at the University of Nevada, Las Vegas.
Mr. Heck has also wavered in his support for Mr. Trump. He disavowed him in the wake of the release of a 2005 “Access Hollywood” tape in which Mr. Trump was caught bragging about leveraging his celebrity status to force himself onto women, and later said only that he wasn’t supporting Mrs. Clinton.
“It didn’t really give a net benefit to him,” Mr. Lee said.
Mr. Lee said GOP Sen. Richard Burr of North Carolina, who was locked in a tight race against Democrat Deborah Ross, and Mr. Rubio were among the incumbents who appeared to be outperforming Mr. Trump. Mr. Burr was first elected in 2004, and Mr. Rubio raised his profile with his presidential run and notably serves on the Senate Foreign Relations and Intelligence committees.
Other closely eyed Senate races some pundits had rated as effectively tossups were in Pennsylvania, Wisconsin and Missouri.
In Pennsylvania GOP Sen. Patrick J. Toomey was battling Democrat Katie McGinty, while in Wisconsin, GOP Sen. Ron Johnson was trying to hold off former Democratic Sen. Russ Feingold in a rematch of their 2010 race.
And in Missouri, GOP Sen. Roy Blunt was seeking re-election against Democratic Secretary of State Jason Kander.
In Colorado Democratic Sen. Michael F. Bennet was expected to win re-election against Republican Darryl Glenn in a race the GOP had hoped to make more competitive.
Republican Sens. John McCain and Chuck Grassley, meanwhile, were expected to win re-election in Arizona and Iowa, respectively, in races Democrats had hoped to put in play.
Still, the sheer number of seats Republicans were defending compared to Democrats, as well as the increased turnout of a presidential year electorate, were thought to give a boost to the Democratic side regardless of who was at the top of the ticket.
“Republicans were playing a lot more defense going into this with respect to the Senate,” said GOP strategist Ford O’Connell. “Frankly, the Democrats did not recruit the strongest candidates they could find.”
The House, meanwhile, stayed in GOP hands, according to multiple network projections. Democrats had needed to net 30 seats to retake control of the lower chamber but were on track to potentially post a net gain of about a dozen or so.
“Everyone knew that the Republicans were going to lose House seats — the only question was only how many,” Mr. O’Connell said.