Ahead of the most important day of campaigning so far for those remaining in the Democrat race for the White House, Republican senators have attempted to revive discredited claims about Ukrainian gas company Burisma, where Joe Biden’s son sat on the board.

As Mr Biden’s campaign picked up steam ahead of the crucial “Super Tuesday” votes after picking up endorsements from three former rivals, the Chair of the Senate Homeland Security and Governmental Affairs Committee, Ron Johnson, announced he was aiming to subpoena documents related to Hunter Biden’s seat on Burisma’s board, according to a letter obtained by CBS News and Reuters.

Hunter Biden’s position with Burisma while his father was the US vice president has been attacked as corrupt by Donald Trump and his Republican allies, but no proof has ever been provided to back up the allegation.

But Mr Trump’s efforts to pressure Ukraine to investigate the Bidens resulted in the president being impeached on abuse of power and obstruction charges in the Democrat-controlled House of Representatives. He was later acquitted by the Republican-controlled Senate.

The renewed effort by Republicans to push Burisma into the spotlight comes as a flood of senior Democrats have swung behind Mr Biden’s presidential bid.

Candidates Amy Klobuchar, and Pete Buttigieg both ended their campaigns on Monday, and have stated their support for Mr Biden, while another former rival, Beto O’Rourke also gave his backing to the Biden campaign.

The swell of endorsements has been interpreted as making Mr Biden the only Democrat in the race who can beat Bernie Sanders – the current front runner, who despite his popularity with many voters is regarded by establishment figures in the party as too socialist, too revolutionary, and altogether too left wing.

Mr Trump has consistently voiced support for Mr Sanders’ campaign, and some Republican voters have even said they will vote for Mr Sanders in their state’s primaries, as they appear to believe that in a Trump vs Sanders 2020 election, Mr Trump would triumph.

This week Mr Trump even suggested the Democratic race was rigged against Mr Sanders.

The efforts by Republicans to bring back the Burisma allegations are a further indication the GOP considers Mr Biden a more serious threat in a head-to-head election.

On Monday, Democrats also warned that the new committee investigation could aid alleged disinformation efforts by Russia and other actors.

Democratic candidates compete on the campaign trail: In pictures Show all 18 1 /18 Democratic candidates compete on the campaign trail: In pictures Democratic candidates compete on the campaign trail: In pictures Jessica Canicosa, a precinct captain for Bernie Sanders, waits to greet caucus voters at Liberty High School in Henderson, Nevada REUTERS Democratic candidates compete on the campaign trail: In pictures Hotel workers at the Bellagio in Las Vegas get to grips with voting papers during the Nevada caucuses AFP via Getty Images Democratic candidates compete on the campaign trail: In pictures A caricature of Bernie Sanders is projected on to a tree during a rally in Las Vegas EPA Democratic candidates compete on the campaign trail: In pictures A woman waits to have a photo taken with Elizabeth Warren during a town hall meeting in Las Vegas REUTERS Democratic candidates compete on the campaign trail: In pictures The threat of coronavirus and other germ-borne illnesses was on some voters' minds at the Democratic caucuses in Henderson, Nevada Getty Images Democratic candidates compete on the campaign trail: In pictures Former vice-president Joe Biden takes a selfie with a voter in Las Vegas ahead of the Nevada caucuses REUTERS Democratic candidates compete on the campaign trail: In pictures Amy Klobuchar changes her shoes backstage after giving a speech in Exeter, New Hampshire AFP/Getty Democratic candidates compete on the campaign trail: In pictures A warmly-wrapped-up dog attends an Elizabeth Warren event at Amherst Elementary School in Nashua, New Hampshire AFP/Getty Democratic candidates compete on the campaign trail: In pictures Bernie Sanders, who romped to victory in New Hampshire against Hillary Clinton in 2016, talks to the media in Manchester Getty Democratic candidates compete on the campaign trail: In pictures Joe Biden was hoping to improve on his poor showing in Iowa in the New Hampshire primary Reuters Democratic candidates compete on the campaign trail: In pictures Elizabeth Warren, renowned for giving time to supporters for selfies, works the crowd at the University of New Hampshire in Durham Getty Democratic candidates compete on the campaign trail: In pictures Joe Biden takes a selfie with a supporter and his child outside a campaign event in Somersworth, New Hampshire on 5 February Reuters Democratic candidates compete on the campaign trail: In pictures Elizabeth Warren and Bernie Sanders quarrel after a confrontation in a TV debate in which Sanders claimed that Warren was not telling the truth about a conversation in which she claimed he had said a woman could not win the presidency on 14 January AP Democratic candidates compete on the campaign trail: In pictures Supporter Pat Provencher listens to Pete Buttigieg in Laconia, New Hampshire on 4 February Getty Democratic candidates compete on the campaign trail: In pictures Pete Buttigieg speaks at a campaign event in Concord, New Hampshire while awaiting the results of the Iowa caucus Reuters Democratic candidates compete on the campaign trail: In pictures Elizabeth Warren is presented with a balloon effigy of herself at a campaign event in Nashua, New Hampshire on 5 February Reuters Democratic candidates compete on the campaign trail: In pictures A Trump supporter rides past a rally for Amy Klobuchar in Des Moines, Iowa on 14 January AP Democratic candidates compete on the campaign trail: In pictures A man holds up a sign criticising billionaires in the presidential race in front of Michael Bloomberg in Compton, Califronia. The former New York mayor skipped the first caucus in Iowa and instead campaigned in California on 3 February Reuters

Intelligence officials have told US lawmakers Russia appears to be engaging in disinformation and propaganda campaigns with the view to help both Mr Trump and promote Mr Sanders, congressional sources told Reuters.

“We need to take every step to ensure the credibility and resources of the US Senate are not used to advance interference efforts by foreign adversaries that seek to undermine our democracy or put our national security at risk,” said Senator Gary Peters, the top Democrat on Mr Johnson’s committee.

Now, Mr Johnson and Senator Chuck Grassley, the Republican chairman of the Senate Finance Committee, are re-igniting their public efforts to assess whether Hunter Biden’s role at Burisma posed a conflict of interest while his father was in the White House.

Mr Johnson told committee members in the letter that he plans to subpoena documents and an interview from Andrii Telizhenko, a former Ukrainian diplomat and consultant for Blue Star Strategies, a lobbying and consulting firm.

Mr Johnson said Blue Star represented Burisma and sought to leverage Hunter Biden’s position in an effort to gain influence with the US State Department.

The letter from Mr Johnson was dated 1 March, a day after Mr Biden’s victory in South Carolina on Saturday.