Despite having previously accepted the US midterm election results, President Donald Trump told reporters on Thursday the elections were "crooked", and he demanded the remaining votes for Alaska go to its incumbent Republican senator.

US President Donald Trump lashed out at "crooked" elections Thursday as the Democrats' midterm steamroller powered ahead, with another potential Senate upset in Mississippi threatening to deepen Republicans' historic defeat.

"Our elections are so crooked, you have no idea," Trump told reporters in the Oval Office, as he denounced the "disgraceful" victory of a democratic socialist in the Los Angeles mayoral race.

His outburst came just two days after Americans handed Democrats control of both chambers of Congress in a sweeping rejection of Trump's presidency and the Republican Party he has remade in his image.

Although Trump initially accepted the midterm results, he has since demanded that votes cast for two minor candidates in Alaska's still-undecided Senate race be handed to the trailing Republican incumbent.

The president's renewed attacks on the electoral process contrast with a political landscape transformed by Tuesday's Democratic landslide.

Democrats are already set to hold at least 54 Senate seats to the Republicans' 44. Their House majority surged past 250 seats on Thursday, the party's biggest in almost two decades.

After Trump warned that he would use his veto power to block Democratic legislation, Senate Democratic leader Chuck Schumer issued a warning of his own.

"If Donald Trump does spend the next year vetoing common sense legislation to lower costs and end corruption, this election – this absolute Republican catastrophe on election night 2026 – will look like a good day compared to what will come down on Republicans in the 2028 election," Schumer said.

Mississippi, Alaska nail-biters The biggest remaining midterms nail-biter was in Mississippi, one of the most solidly Republican states in the nation, where against all expectations a 43-year-old Black candidate, Scott Colom, appeared to be on the cusp of winning a US Senate seat.

By afternoon, Colom's lead ahead of incumbent Republican Cindy Hyde-Smith was inching up at 1,724 votes and only a handful of ballots left to count. The final result could take days to come.

Colom would be the first Democrat to win a Senate seat from the southern state since 1982 – and the first African American to do so since 1874.

In Alaska, Democrat Mary Peltola was also just ahead of the pack with 48.2 percent of the vote, followed by Republican incumbent Dan Sullivan with 47.9 percent.

Trump's Truth Social outburst referred to a fringe candidate also named Dan Sullivan, whom Republicans accuse of being a "decoy" aimed at siphoning votes from their candidate, and another minor contender Gerald Heikes, who is registered as a Republican.

Trump posted that all the votes for the "Fake" Sullivan and Heikes "should be immediately given to the REAL Dan Sullivan". "There is no other fair solution!"

Making history If Colom wins in Mississippi, he would join a host of other new faces smashing racial and cultural barriers in the midterms.

Keisha Lance Bottoms, the great-great-granddaughter of a slave, became the first Black female governor of Georgia – and in the country.

And Michigan's Abdul El-Sayed beat a Trump ally to become the first Muslim to enter the US Senate.

Tuesday's midterms also featured wins for Democrats in political territory where Trump had previously been so dominant that it was all but written off by the left.

James Talarico broke a 38-year hoodoo by winning a Senate seat in ultra-Republican Texas and Adam Hamilton did even better, becoming the first Democratic senator from Kansas in 94 years.

Republican congressman Warren Davidson, in Ohio, said Trump's months-long war against Iran and global trade wars had lost the election for the party. Young voters in particular, he said, felt "betrayed".