WASHINGTON (AFP) — President Joe Biden's Democrats retained control of the US Senate on Saturday, a remarkable midterm 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 on Saturday US networks 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.

Biden, due to meet Chinese President Xi Jinping on the sidelines of the G20 summit on Monday, said control of the Senate would bolster his position in the talks.

"I know I'm coming in stronger," he said of the midterms' impact.

One Senate race remains up in the air — a runoff in Georgia set for 6 December, 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.

Call to 'come together'

Speaking minutes after the projections were announced, Schumer 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.

Schumer underscored that the Democrats' win would ensure a "firewall" against moves by Republicans in Congress to further curtail abortion rights — a key issue in the midterms.

But the Senator for New York also urged the two parties to "try to come together" to end "divisive negativity."

The two parties had been neck-and-neck at 49 seats each after Democrat Mark Kelly was projected to win a tight Senate race in Arizona on Friday evening.

The former astronaut beat out challenger Blake Masters, who has not yet conceded defeat and was backed by Trump.

Trump's response to the Arizona result was to double down on unfounded claims of ballot rigging, posting on his Truth Social platform that the Democrat's victory was a "scam" and the result of "voter fraud."

The underwhelming outcome for Republicans has prompted a bout of internal finger-pointing, with targets including Trump, party leaders, and campaign messaging.

US media on Saturday cited a letter circulated by three Republican senators calling for the postponement of party leadership elections currently scheduled for the middle of next week.

"We are all disappointed that a Red Wave failed to materialize, and there are multiple reasons it did not," the letter said.

"We need to have serious discussions within our conference as to why and what we can do to improve our chances in 2024," it added.

After the Senate result was projected, Republican Senator from Missouri Josh Hawley called in a tweet for the party to "build something new."

"The old party is dead. Time to bury it."