Washington (CNN) There may be no endorsement, but this time, Barack Obama didn't try to talk Joe Biden out of running for president.

As Biden finally jumped into the 2020 presidential race Thursday, his decision not to challenge Hillary Clinton for the Democratic nomination in 2016 is among the factors fueling his candidacy now.

For the last four years, Biden has never fully reconciled with his choice. It's a decision Obama and his top aides helped steer him toward, considering Biden was grieving the death of his son, Beau, and Obama had already promised his support to Clinton.

But this time around, Obama didn't urge Biden to stay on the sidelines and watch as the largest and most diverse field in the history of the Democratic Party took shape.

Instead, during a handful of conversations in recent months, people close to both men say Obama listened as Biden explained why he believed he was uniquely suited to try and stop the re-election of President Donald Trump. They talked about family concerns, people familiar with the conversations said, with Obama acting more as a close friend than a former president.

"He would never tell Joe not to run," said one Obama confidant, speaking on condition of anonymity to discuss private conversations. "Does he think Biden will win the primary? He doesn't know the answer any more than the rest of us do."

Yet Biden's entry into the 2020 campaign -- a candidacy that will effectively be seen as a third Obama term -- will bring fresh scrutiny to their eight years in office in the context of today's more progressive Democratic base. Biden will point to his White House record with Obama far more than the nearly four decades he spent in the Senate.

One challenge facing Biden will be whether he's seen as Obama's rightful heir or whether that position belongs to a newer figure in the party. That will only become clear after the Democratic primary fight unspools.

One thing Obama does know? Endorsements seldom work -- even from a former president who is one of the most popular figures in the Democratic Party. And Biden didn't directly ask for one knowing full well he must win this primary battle on his own.

"I asked President Obama not to endorse and he doesn't want to," Biden told reporters Thursday in Delaware. "Whoever wins this nomination should win this on their own merits."

While Obama and Biden have what aides describe as an "exceptionally close" friendship, they actually talk by phone only sporadically and meet face-to-face even less frequently.

Yet even as some former members of the Obama administration have wondered aloud about the wisdom of a Biden candidacy, Obama has been reluctant to criticize his former vice president even in his private conversations.

Obama has met with nearly every Democratic presidential hopeful in recent months, but his discussions with Biden were far more personal in nature, people familiar with the conversations say, because he knows Biden and his family more intimately. But Obama was less of a sounding board for a potential candidacy for Biden, as he was for some other candidates.

In fact, Obama was largely kept apprised of Biden's plans through their respective aides, particularly because their record from eight years in the White House will suddenly become front-and-center in the Democratic primary. And Obama made clear in a rare statement issued from his office on Thursday that he thinks highly of Biden.

"President Obama has long said that selecting Joe Biden as his running mate in 2008 was one of the best decisions he ever made," said Katie Hill, an Obama spokeswoman. "He relied on the Vice President's knowledge, insight, and judgment throughout both campaigns and the entire presidency. The two forged a special bond over the last 10 years and remain close today."

Relationships between president and vice president are, by their very nature, complicated.

By all accounts, Obama and Biden have a far stronger bond than any recent White House partnership, considering the post-presidency tensions between George W. Bush and Dick Cheney and the turmoil between Bill Clinton and Al Gore in their final term.

While Biden described himself earlier this month as an "Obama-Biden Democrat," it was notable that he didn't mention Obama a single time in his announcement video on Thursday. Instead, he focused on the president currently in the Oval Office.

"If we give Donald Trump eight years in the White House, he will forever and fundamentally alter the character of this nation, who we are and I cannot stand by and watch that happen," Biden said. "The core values of this nation, our standing in the world, our very democracy, everything that has made America America's at stake."