The Wisconsin senator Ron Johnson, a hardline Trump supporter once described as “what you get when QAnon and the Tea Party have a baby”, has reportedly decided to seek a third term, a step he once promised not to take.

Two Republicans confirmed Johnson’s plan to the Associated Press and said he could announce as soon as early next week. Johnson did not comment.

Both parties are likely to welcome the news, given Johnson’s emergence as a leading promoter of both Donald Trump’s lies about election fraud and Covid-19 misinformation.

In a Republican party dominated by Trump, who has endorsed Johnson, a third run would avoid a chaotic primary.

Among Democrats, Johnson is seen as beatable in a November contest which will help decide control of a Senate split 50-50 and controlled via Vice-President Kamala Harris.

With Republicans favoured to take back the House, Democrats are desperate to hold the Senate, not least to protect Joe Biden’s chances of naming at least one justice to a supreme court skewed 6-3 in favour of conservatives after Trump’s time in power.

Earlier this month, Brandon Scholz, a Republican operative, told the Hill: “I think you will find almost every Republican in Wisconsin and outside of Wisconsin wanting Ron Johnson to run because of what’s at stake, and that’s the majority of the Senate for Republicans. If he doesn’t run, that makes it more difficult.”

A Wisconsin Democrat, Ben Nuckels, said: “Ron Johnson is what you get when QAnon and the Tea Party have a baby. And I hope that he does run. His candidacy makes the race far more competitive for Democrats. If Republicans want to see him run, I’ll agree with them on that.”

In 2016, Johnson pledged not to run a third time, a promise rescinded when Democrats took Congress and the White House.

Wisconsin is a battleground state. Joe Biden won by fewer than 21,000 votes in 2020, after Trump won a similarly thin victory in 2016. In midterms, the party that does not hold the White House generally makes gains. For example, in 2010, under Barack Obama, Republicans picked up 63 House seats and six in the Senate.

Johnson rose out of the Tea Party movement stoked that year by opposition to Obama’s healthcare reform and by rightwing donors. He defeated an incumbent Democrat, Russ Feingold, then beat him again in 2016.

Johnson is now one of Trump’s loudest defenders, standing by him after the attack on the US Capitol last year. The senator has espoused conspiracy theories about electoral fraud and the Capitol attack. On the legalistic side of Trump’s attempt to remain in power, Johnson planned to object to results in Arizona but changed his mind after the events of 6 January.

In a statement, however, he said he still refused “to dismiss the legitimate concerns of tens of millions of Americans who have lost faith in our institutions and the fairness of our electoral process”.

Newspapers called for him to resign. The Wisconsin State Journal said: “Johnson’s last-minute change of heart may be viewed by some as proof of his conscience. Yet it is more accurate to view his flip-flopping … as a hit-and-run driver fleeing the scene of an accident because the driver hears sirens in the distance – only to come back to the scene and flick an insurance card out the window and keep on driving.”

Referring to Johnson and Republicans who went through with objections to electoral college results, the paper said: “These men are cowards.”

Johnson has also been a loud voice for unproven Covid treatments, accusing federal agencies of failing to promote drugs approved early in the pandemic and opposing public health measures including vaccine mandates.

Earlier this week, Dr Rob Davidson, leader of the Committee to Protect Healthcare, an advocacy group, “begged” Twitter to “look at the last two weeks” of Johnson’s feed “and shut him down like you did Marjorie [Taylor] Greene”.

Greene, an extremist congresswoman from Georgia, was removed from Twitter last week, for spreading Covid misinformation.

Johnson “has at least five strikes of Covid mis/dis-information”, Davidson said, adding: “Feeds like his undermine our ability to save lives and end the pandemic.”

Johnson has protested Twitter decisions concerning tweets about Covid.

Democrats running to face Johnson include the lieutenant governor, Mandela Barnes; Alex Lasry, an executive with the Milwaukee Bucks NBA team; and the state treasurer, Sarah Godlewski.

On Friday, Barnes said: “Ron Johnson has been a failure and Wisconsin voters know it. The only people cheering Johnson’s decision are the wealthy special interests and big donors who have made a killing during his time in Washington.”

Also on Saturday, John Thune, a member of Senate Republican leadership, said he would run for a fourth term. His state, South Dakota, is not remotely as competitive as Wisconsin.