Washington (AFP) - President Joe Biden's Democrats edged closer to retaining control of the US Senate on Friday, as Donald Trump prepared to declare his bid for the White House in 2024.

Democratic Senator Mark Kelly won re-election in Arizona, three television networks announced. His victory gives Democrats 49 Senate seats, one short of securing a majority, with Nevada still counting votes and Georgia's contest headed to a December 6 runoff.

If the two parties split the remaining two seats, the Democrats retain control of the Senate because Vice President Kamala Harris will cast the tie-breaking vote.

Blake Masters, the Republican rival to Kelly, did not immediately concede defeat in Arizona, and late Friday Trump posted on his social media account that some voting machines in Arizona didn't work and the result is "a scam and voter fraud... Do election over again!" 

Trump will announce next week that he is taking another shot at the presidency with a White House run in 2024, his longtime advisor Jason Miller said Friday.

The divisive former president, who will be 78 when the next election is held, has been hinting at another presidential run while campaigning for Republican candidates ahead of this week's midterm elections, and said he will make a "very big announcement" on Tuesday.

"President Trump is going to announce on Tuesday that he is running for president," Miller told former Trump aide Steve Bannon on his popular "War Room" podcast.

"It's gonna be a very professional, very buttoned-up announcement," he added.

Miller said Trump told him, "there doesn't need to be any question, of course, I am running."

Trump's candidacy will mark his third shot at the presidency, including his loss to Joe Biden in 2020. After that defeat, he promoted baseless claims of fraud, including those that led to an unprecedented riot at the US Capitol in Washington.

Seats flipped

Trump's big announcement in Florida comes after a disappointing run for several candidates he backed in the midterms. 

Some of his hand-picked favorites even lost Republican-held seats to Democrats.

In Pennsylvania, Democrats flipped a highly-prized US Senate seat with constant attacks on Trump-endorsed celebrity doctor Mehmet Oz, who had never held public office before and lived mostly in New Jersey.

Trump had hoped to ride a Republican "red wave" that would prime him for another presidential run but the party achieved a much smaller victory than had been predicted.

With 211 seats so far, Republicans appear poised to secure a slim majority in the 435-seat House of Representatives. However, control of the Senate may come down to a December 6 runoff in the southeastern state of Georgia.

The former president's major media ally -- the powerful empire of conservative billionaire Rupert Murdoch -- even turned on him in the wake of the polls.

Pointing to the party's disappointing midterms showing, The Wall Street Journal, the flagship of Murdoch's News Corp, declared in an editorial on Thursday that "Trump Is the Republican Party's Biggest Loser." 

The cover of the tabloid New York Post depicted Trump on a precarious wall as "Trumpty Dumpty" who "had a great fall."

Nevertheless, more than 100 Republican candidates who challenged the 2020 presidential election results won their respective races.

Trump's early entry into the race would appear designed in part to fend off possible criminal charges over taking top secret documents from the White House, his efforts to overturn the 2020 election and the attack on the US Capitol by his supporters on January 6 last year.

It may also be intended to undercut his chief potential rival for the Republican presidential nomination, Florida Governor Ron DeSantis, who emerged as one of the biggest winners in Tuesday's midterms

Trump is still tangling with the congressional January 6 committee investigating the attack on the Capitol. Earlier Friday, his lawyers challenged a subpoena from the committee, saying Trump has "absolute immunity" and will not testify next week. 

The subpoena is "invalid, unlawful, and unenforceable," the lawyers said in the lawsuit, because Trump "has absolute immunity from being compelled to testify before Congress... regarding his actions as head of a co-equal branch of government."