US President Joe Biden's Democrats have retained control of the Senate, a remarkable midterms election result that defied predictions of a Republican win over both houses of Congress.

Midterms traditionally deliver a rejection of the party in power, and with inflation surging and Biden's popularity in the doldrums, Republicans had been expecting to ride a mighty "red wave" and capture the Senate and the House of Representatives.

But the wave never got much beyond a ripple and US networks have called the key Senate race in Nevada for Democrat incumbent Catherine Cortez Masto, giving the party the 50 seats it needs for an effective majority.

The win clinches Democratic control in the Senate as Vice President Kamala Harris can cast the tie-breaking vote if the upper chamber is evenly split 50-50.

"I feel good and I'm looking forward to the next couple years," Biden said of the result, speaking at a meeting of Southeast Asian leaders in Phnom Penh on Sunday.

One Senate race remains up in the air -- a runoff in Georgia set for December 6, in which the Democrats could add to their majority.

The result in the House of Representatives still hangs in the balance, and while Republicans are slightly favored to take control, it would be with a far smaller majority than they had envisaged going into Tuesday's election.

Senate Majority Leader Chuck Schumer was quick to ring in his party's win, tweeting the result was a "vindication" of Democrats' achievements.

Speaking minutes after the projections were announced, he said the result showed Americans "soundly rejected the anti-democratic, authoritarian, nasty and divisive direction the MAGA Republicans wanted to take our country," referring to former president Donald Trump's "Make America Great Again" movement.

Trump was omnipresent on the campaign trail, putting his thumb on key Republican primaries and holding rallies nationwide, during which he repeated his baseless claims of fraud in the 2020 race.

While more than 100 Republican candidates who challenged the 2020 presidential election results won their races, according to US media projections, some of Trump's hand-picked candidates underperformed and the Republicans' poor showing overall was a damaging political blow.

Trump is set to declare his 2024 White House bid on Tuesday -- an announcement he had planned as a triumphant follow-on to an expected crushing election victory by the party he still dominates.

Maintaining control of the Senate means Biden and the Democrats will retain key leverage in legislative debates, particularly in domestic and foreign spending policy. (AFP)