After refusing to make any public displays of support for Joe Biden as he sought the 2020 Democratic nomination, Barack Obama was reportedly working behind the scenes to convince Bernie Sanders to withdraw from the race, drawing him into "at least four long conversations" about his exit, according to reports.

Until this week, Donald Trump's two-term predecessor declined to publicly endorse Mr Biden, his former vice president, and his endorsement arrives one day after Mr Biden's attempt to form an allegiance with Mr Sanders in a bid to secure his legion of supporters who remain unconvinced that the former vice president is willing to adopt a progressive platform.

But while Mr Sanders won unprecedented popular vote contests in the first four primary states, a first for any Democratic contender, Mr Obama had reportedly turned down requests to be more involved with Mr Biden's campaign, as insiders had hoped his public support would end Mr Sanders's surge, The New York Times reported.

As Mr Biden emerged as a likely nominee, Mr Obama reportedly spoke with Mr Sanders to "accelerate the endgame" in late March by seeking his departure and clearing a path for Mr Biden, The Times reported. They spoke at least three other times, The Times said.

Mr Obama's endorsement on Tuesday cashes in what remains of Mr Biden's political capital following high-profile endorsements from Democratic figures and his chief rival.

After Mr Biden's streak of primary wins, Mr Trump baited the Democratic establishment by expressing sympathy for Mr Sanders and attacking Elizabeth Warren, likely as an attempt to undermine the legitimacy of the race among his rivals.

In a recent White House briefing, he said: "I'll tell you, it does amaze me that President Obama hasn't supported Sleepy Joe ... It just hasn't happened. When is it going to happen? When is it going to happen? Why isn't he? He knows something that you don't know, that I think I know, but you don't know. So it'll be interesting."

Both campaigns used older video and audio clips of Mr Obama voicing support for both men in their respective ads.

Mr Sanders ultimately left the race last week after the Wisconsin primary election, another in a string of primaries held in the midst of the coronavirus pandemic, forcing voters to crowd at the polls after last-minute attempts to postpone the election or allow late absentee ballots were blocked by Republicans and conservative judges.

On Monday, he appeared in a livestream video with Mr Biden to offer his endorsement and announce the formation of "policy working groups" relying on experts from both men's campaigns.