In total, 12 Republican-held seats and two Democratic-held seat had been in play
Republican Senator Cory Gardner was defeated on Tuesday in Colorado by former Governor John Hickenlooper, giving the Democrats their first victory of an election battle in which they are attempting to win control of the US Senate.
That win was quickly offset, however, by the loss of Democratic Senator Doug Jones in Alabama, while veteran Republican Senators Lindsey Graham and John Cornyn fended off Democratic challenges in South Carolina and Texas.
Republicans also held onto an open seat in Kansas, where Republican Roger Marshall was declared the winner over Democrat Barbara Bollier.
Control of the Senate may not become clear for some time. Final results from multiple contests may not be available for days, and in at least one case, months. One of two Georgia Senate races was projected to go to a January runoff between Republican Senator Kelly Loeffler and Democrat Raphael Warnock.
Voters are also deciding whether to end the political career of moderate Susan Collins of Maine, among other embattled Republican senators.
In total, 12 Republican-held seats and two Democratic-held seat had been in play, based on a Reuters analysis of three nonpartisan US elections forecasters - the Center for Politics at the University of Virginia, the Cook Political Report and Inside Elections.
Gardner, a first-term Republican long seen as his party's most vulnerable Senate incumbent, lost to Hickenlooper in a formerly Republican state where demographic changes have increasingly favored Democrats in recent years, according to projections by television networks and Edison Research.
Jones, the most vulnerable Democrat, lost as expected to challenger Tommy Tuberville in the Republican stronghold of Alabama.
To win the majority in the Senate, Democrats need to pick up only three Republican seats if Joe Biden is elected president and Senator Kamala Harris wields the tie-breaking vote as vice president. Republicans now hold a 53-47 majority.
Graham, the Senate Judiciary Committee chairman who was massively out-fundraised by Democrat Jaime Harrison, rode to victory after presiding over the Senate confirmation of US Supreme Court Justice Amy Coney Barrett.
Cornyn was declared the winner against challenger MJ Hegar in a state that had appeared to be drifting toward Democrats. Republican Senator Majority Leader Mitch McConnell won re-election in Kentucky, as expected.
All told, 35 of the Senate's 100 seats were up for election.
"There are dogfights all over the country," McConnell, the top Republican in Congress, said at a campaign stop earlier in the week. He described the possibility of Republicans holding onto the Senate majority as a "50-50 proposition."
Those odds appear optimistic, based on the three forecasters that expect Democrats to emerge from the election with control of Congress for the first time in a decade. Democrats were projected to maintain control of the 435-seat House of Representatives.
Democrats are hoping to usher in a new political era in Washington if Biden also wins.
Although likely to fall short of a 60-vote filibuster-proof majority, Democratic Senate control would also help stymie a second Trump term.
Gardner was among more than half a dozen first-term party incumbents in states also including Alaska, Arizona, Georgia, Iowa, Montana and North Carolina. Democrat Gary Peters is on the defensive in Michigan.
Delays Expected In Results
Results from some races are not likely to be known until after Election Day, due to this year's unprecedented volume of mail-in ballots and possible runoff elections in four races.
Delayed results could occur in Arizona and Maine, where Democrats are strongly favored to flip Republican seats.
Final results from a four-way Maine contest among Collins, Democrat Sara Gideon and two independent candidates could be delayed for 10 days to two weeks if no candidate wins an outright majority and the race is forced into an automatic runoff under the state's ranked-choice voting system, according to a state election official.
Maine voters can rank candidates in order of preference. With no clear winner on election night, the contest would enter a series of elimination rounds in which lower-ranked candidates drop out until a victor emerges.
In Arizona, Democrat Mark Kelly appeared poised to unseat Republican Senator Martha McSally. But county authorities have up to 20 days to review election results. McSally's failed 2018 election contest against Democratic Senator Kyrsten Sinema took six days to produce a winner.
In Michigan, where Peters could be vulnerable to an upset by Republican John James, state election officials warned that final results may not be available until Friday.
The outcome of a tight race in Montana between Republican incumbent Steve Daines and Governor Steve Bullock may not be known until Wednesday, according to state election and Democratic Party officials.
If Democrats do emerge from the election with Senate control, Senate Democratic leader Chuck Schumer has vowed to let nothing stand in their way. "Nothing is off the table," he said.