Former President Barack Obama will endorse Joe Biden on Tuesday, a source familiar with the plans tells CNN.

Obama will make the endorsement of his former vice president in a video message, offering why he believes Biden is the candidate needed at this moment of crisis in America, a person close to the former president says.

The endorsement reunites the former running mates and positions Obama, whose endorsement of Biden was seen as a forgone conclusion once Vermont Sen. Bernie Sanders dropped out of the race last week, to be one of Biden's most powerful surrogates in the race against President Donald Trump.

It also marks the public re-emergence of Obama into the political arena. The former president kept a low public profile throughout much of the Democratic nomination fight, but he was active behind the scenes.

Obama in November said he had spoken to all of the candidates during the campaign, often giving them advice both before they announced their bid and after they dropped out. Advisers to the former president said his advice was often the same: Consider why you believe you should be president, what impact it will have on your family and whether you can actually win.

"We have a field of very accomplished, very serious and passionate and smart people who have a history of public service," Obama said at a private fundraiser in late 2019. "Whoever emerges from the primary process, I will work my tail off to make sure they are the next president."

Obama had long said he would get behind the eventual nominee, but the former president's deep affection for Biden was well known to all those who ran against the former vice president this year. Biden even jokingly posted an homage to Obama on "Best Friends Day" in 2019, featuring a friendship bracelet that featured the names "Joe" and "Barack".

That relationship, though, did not keep Obama from giving counsel to other candidates.

As Sanders worked to determine the future of his campaign, Obama and the Vermont senator spoke multiple times, a source familiar with the conversation told CNN. Sanders endorsed Biden on Monday in a livestream, pledging to help him defeat President Trump.

"We need you in the White House. I will do all that I can to see that that happens, Joe," Sanders said to Biden.

In the few moments where Obama did speak out about the race, the former president warned voters of worrying too much about the bruising nature of a primary, reminding them that he and then-Sen. Hillary Clinton had a tough primary in 2008.

And the former president told the candidates to avoid going so far left that they go beyond where voters actually are.

"My one cautionary note is I think it is very important for all the candidates who are running at every level to pay some attention to where voters actually are," Obama said in November, specifically saying he doesn't think candidates should be "diluted into thinking that the resistance to certain approaches to things is simply because voters haven't heard a bold enough proposal."

It is this period of the campaign, however, when some supporters of other candidates, especially Sanders backers, feel like their hopes were dashed once their candidates dropped out, where Obama's team believes the former president can be most useful.

"He is uniquely positioned to galvanize the party around the nominee and increase voter turnout so that a Democrat is successful in the general election," Valerie Jarrett, a longtime Obama adviser, told CNN in March.

Jarrett said that they believed it was "important" to let all the candidates "stand on their own two feet" but that once a nominee was selected, Obama would use the support he enjoyed from a "a broad spectrum of the Democratic Party, both the progressive and moderate wings" to bring the factions together.

This story is breaking and will be updated.