A victory in Nevada is expected to keep Democrats in control of the U.S. Senate.
Democratic incumbent Catherine Cortez Masto received 48.7 percent of the vote, narrowly ahead of Republican former state attorney general Adam Laxalter with 48.2 percent.
Ms Cortez Masto, 58, has campaigned heavily on abortion and also criticized Mr Laxalt’s ties to big oil companies, which have posted record profits in recent months new highs.
Mr. Laxalt had tried to link Ms. Cortez Masto to President Joe Biden’s economic policies, blaming them for inflation and rising fuel prices.
The day before the election results were announced, Democrat Mark Kelly declared victory in Arizona, defeating his Republican challenger Blake Masters, who, like Mr Laxalter, was backed by former President Donald Trump.
That means Democrats have 50 seats in the Senate.
Democratic Vice President Kamala Harris can break the deadlock in the 100-member House, which means the party can win for President Biden.
This will be especially important if a seat on the US Supreme Court – which currently has a 6-3 conservative majority – opens up in the final two years of Mr Biden’s term.
Democrat Mark Kelly wins Arizona Senate race, overall control still hanging in the balance
Surge in Republican support for Trump fails to take off
Donald Trump may soon be yesterday’s news as US right-wing media turns to Ron DeSantis
President Biden told reporters in Cambodia ahead of the East Asia summit that he was “very happy” with the election results.
“We’re focused on Georgia right now. We feel good about where we are. And I know I’m a self-righteous optimist. I understand that,” he said.
“Again, I’m not surprised by the turnout. I’m very pleased. I think it reflects the quality of our candidates.”
He added: “I feel good. I’m looking forward to the next few years.”
“We’ve done a lot and we’ll do more for the American people,” Senate Majority Leader Chuck Schumer said.
“The American people reject – outright reject – the anti-democratic, authoritarian, nasty and divisive direction the MAGA Republicans want to take our country.”
No matter where you get the podcast, you can click to subscribe to Sky News Daily
The last state to be decided is Georgia, where Democratic Sen. Raphael Warnock faces Republican Sen. Herschel Walker on Dec. 6.
However, the 435-seat House of Representatives is still in the balance.
Republicans have the advantage, but votes are still being counted in several races, including many in liberal-leaning California.
If Democrats manage to secure a victory in the House, it would mean full control of Congress by Democrats – and yet another chance to advance Mr Biden’s priorities.
The party still lacks the 60 votes in the Senate it needs to push through a variety of major legislative reforms.
Surprise performance by Democrats will make Republicans reassess relationship with Trump
The midterm elections are said to be a referendum on the success of the incumbent president and his first two years in the White House.
With President Biden’s approval ratings sluggish, inflation soaring, and nearly every mouthful gaffe, the Democratic candidate’s success means nothing when normal rules are applied.
But this is no ordinary election.
One way to interpret it is to see it as a referendum on the far-right politics of “Make America Great Again” candidates, who in some cases were recruited and endorsed by former President Donald Trump.
Take Adam Laxalt, a Republican who lost the decisive Senate race in Nevada and was a fully-funded member of Trump’s campaign-denying lies.
Republican Senator Lindsey Graham said he “has no mathematical way” to fail and that he may have forgotten to include discerning voters in his equation.
Across the country, up and down the ballot — with few but not many exceptions — voters condemned Trumpism.
From celebrity TV doctor Mehmet Oz beaten in Pennsylvania to anti-abortionist Yesli Vega beaten in a key Virginia race, this wasn’t the red wave Republicans were expecting. A decision must now be made whether to continue the former president’s alliance with the GOP
Read more from Martha