Democrats picked up one seat in Colorado on Tuesday, and Republicans held off a challenge in Texas in the battle for control of the U.S. Senate.
Republicans currently hold a majority of 53 seats in the 100-seat chamber. The battle spans 14 competitive races, though the final outcome may not be clear for some time.
Democrats need to win four seats to take a majority, or three if Joe Biden wins the White House, giving a Vice President Kamala Harris the tie-breaking vote.
A Democratic victory could lead to a new era in U.S. politics, if the party also captures the White House and holds onto the U.S. House of Representatives.
Here are the results so far, based on projections by television networks and Edison Research:
Democratic challenger John Hickenlooper defeated incumbent Republican Senator Cory Gardner.
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Gardner, a former U.S. representative who entered the Senate in 2015, was among the most vulnerable Senate Republicans partly because of his allegiance to Trump in a state that has gone Democratic in the last three presidential elections.
Hickenlooper is a former two-term governor and 2020 presidential hopeful. He raised far more money than Gardner and consistently led in opinion polls.
Veteran Republican Senator John Cornyn held off a challenge from Democrat M.J. Hegar.
Texas, once a Republican stronghold, has grown increasingly competitive in recent years as the population has grown more diverse and Donald Trump's polarizing presidency has alienated suburban women.
The state has become a surprise battleground in this year's presidential race, where opinion polls have shown Trump narrowly leading Democrat challenger Joe Biden.
The Senate race ended up being less competitive, despite Hegar's strong fundraising. Edison Research projected Cornyn to win re-election shortly after polls closed.
Cornyn has been in the Senate since 2002.
Here are other races that are likely to determine which party will control the Senate:
Freshman Republican Senator Dan Sullivan is favored to hold onto his seat in a closely contested election battle against Al Gross, an independent who is running as the Democratic Party nominee.
The latest polling shows Sullivan with a slim 3 percentage point lead over Gross, according to the poll-tracking website FiveThirtyEight.com. But polling data has shifted the lead back and forth in recent weeks. Gross has also out-fundraised Sullivan by a factor of more than 2-to-1.
Republican Senator Martha McSally lagged Democratic challenger Mark Kelly in fundraising and trails him by an average of nearly 6 percentage points in opinion polls, according to the campaign tracking website RealClearPolitics.com.
McSally, a former U.S. representative and U.S. Air Force combat pilot, was appointed to the seat once held by the late Republican Senator John McCain after losing her 2018 Senate bid to Democrat Kyrsten Sinema. Kelly, a former astronaut and U.S. Navy combat pilot, has been leading McSally in opinion polls for more than a year.
First-term Republican Senator David Perdue, a wealthy businessman who promotes himself as a Trump ally, trails Democratic challenger Jon Ossoff by a razor-thin 1 percentage point margin, according to RealClearPolitics.com.
Ossoff, an investigative journalist and media executive, ran a powerful campaign for a U.S. House of Representatives special election in 2017 but ultimately lost. Democrats believe he could oust Perdue, partly as a result of public dislike for Trump's handling of the COVID-19 pandemic that has hit Georgia hard.
Perdue's is one of two Georgia Senate seats that could be settled by a January runoff if no candidates receives more than 50% of the vote.
In an election fluke caused by the retirement of former Republican Senator Johnny Isakson, both of Georgia's U.S. Senate seats are up for grabs on Tuesday.
Republican Senator Kelly Loeffler, who was appointed as Isakson's replacement last year, is running in a multi-party, multi-candidate "jungle primary" special election, featuring a powerful Republican opponent, Representative Doug Collins.
The contest is widely expected to end in a January run-off election. But as Loeffler and Collins battle each other for Republican votes, Democrat Raphael Warnock leads in the polls and had more cash on hand than either Republican as the race entered its final weeks.
A close contest between Republican Senator Joni Ernst and Democrat Theresa Greenfield appeared to tighten in the Republican's favor in the final days of the campaign. Ernst leads Greenfield, an urban planner and real estate developer, by an average of 2 percentage points, according to RealClearPolitics.com.
Greenfield has accused Ernst of being a rubber stamp for Trump and not taking the COVID-19 pandemic seriously enough. Ernst, who is trailing Greenfield in campaign money, has sought to use her role in U.S. Supreme Court Justice Amy Coney Barrett's confirmation to appeal to conservative-leaning voters.
Not since 1932 has a Democrat won a U.S. Senate seat in Kansas, one of the country's most reliably Republican states. But this year, Democratic state Senator Barbara Bollier and Republican Representative Roger Marshall are locked in a tight race for the seat of Republican Senator Pat Roberts, who is retiring.
Marshall leads Bollier by a popular vote margin of just over 5 percentage points, according to FiveThirtyEight.com, even though the Democrat has raised more than four times in campaign donations.
Four-term Republican Senator Susan Collins, a New England moderate long known for her independence, has seen her popularity flag among voters amid criticism that she failed to be a moderating force in the Senate during Trump's presidency.
Her Democratic challenger, Maine House of Representatives Speaker Sara Gideon, leads Collins in opinion polls and has massively out-fundraised the Republican. But the race has tightened in Collins' favor in recent days, and voters' second choices could be taken into account under Maine's unique runoff system if no candidate has more than 50% of the vote.
Republican Senator Steve Daines is running neck-and-neck against two-term Governor Steve Bullock, a former presidential candidate who has branded himself as an independent-minded Democrat. Daines, a former congressman and software executive, is known as a reliable conservative and has touted his ties to Trump.
Bullock was a late entry, jumping into the race in March. But he managed to raise funds quickly and shows signs of holding a small lead over Daines in the final stretch.
North Carolina has erupted into one of the most dramatic Senate races in the country, with Democrat Cal Cunningham's candidacy tripped up by a sex scandal after he had dominated the campaign for months.
Once among the Democratic Party's strongest Senate candidates, Cunningham has seen his double-digit lead over Republican Senator Thom Tillis erode to low single digits. But it is not clear whether a sex scandal in the age of Trump can fully overcome the tide of political polarization that has largely run against Tillis up to now.
Senator Lindsey Graham, one of Trump's closest allies in Congress, was last re-elected to the Senate in 2014 with more than 55% of the vote. But this year, he is favored to win by only a small margin against Democrat Jaime Harrison, who is running a powerful campaign backed by staggering fundraising numbers.
Recent polls show Graham with a slight lead over Harrison, based partly on his role in overseeing Barrett's confirmation. Once a Trump critic, Graham faces skepticism among conservatives over his recent conversion to Trump ally, while the loss of his former maverick persona has disappointed moderates, analysts say.
DEMOCRATIC HELD SEATS
Senator Doug Jones, considered the most vulnerable Democrat in the Senate, won election to the seat that Republican Jeff Sessions vacated to become Trump's attorney general in 2017. In a state normally considered safe for Republican candidates, Jones bested former Alabama Supreme Court Chief Justice Roy Moore, whose campaign was snarled by allegations of sexual misconduct involving young women.
The Democrat now faces former Auburn University football coach Tommy Tuberville, a Republican who defeated Sessions' attempted comeback earlier this year. Tuberville has seen his lead widen in the closing weeks of the campaign, despite more aggressive fundraising by Jones.
Democratic Senator Gary Peters has a small lead over Republican challenger John James in a state that represents a major battleground for Trump's re-election bid.
The race leans in Peters' direction, according to analysts. But James, a Michigan businessman, has run a strong campaign, raising Republican hopes that he could score an upset victory over the incumbent on Election Day.