Explained | Why Obama hasn't endorsed Harris

Many of the marquee names in Democratic politics began quickly lining up behind Vice President Kamala Harris on Sunday, but one towering presence in the party was conspicuously and purposefully absent: Barack Obama.

The former president has not yet endorsed Harris; in fact, he did not mention her once in an affectionate -- if tautly written -- tribute to President Joe Biden that was posted on Medium shortly after Biden decided to bow out Sunday.

"Joe Biden has been one of America's most consequential presidents, as well as a dear friend and partner to me," wrote Obama, who chose Biden as his running mate in 2008 because he said at the time that he wanted an older, more experienced running mate with "gray in his hair" and limited future presidential ambitions.

"We will be navigating uncharted waters in the days ahead," Obama wrote in the post. "But I have extraordinary confidence that the leaders of our party will be able to create a process from which an outstanding nominee emerges."

Republicans interpreted that as a snub. But people close to Obama, who has positioned himself as an impartial elder statesman above intraparty machinations, said not to read too much into it -- and had no alternate candidate in mind when he made the decision not to immediately endorse Harris.

Obama adopted an identical stance four years ago when Biden's aides pressured him to endorse early in the Democratic primaries before Sen. Bernie Sanders dropped out. (Obama's favored phrase back then was "I don't want to thumb the scale.") Endorsing too early now would also be a political mistake -- fueling criticism that Harris' nomination, should it come, was a coronation rather than the best possible consensus under rushed circumstances, they said.

Instead, Obama sees his role as helping to quickly "unite the party once we have a nominee," a person familiar with his thinking said.

But there are other more personal considerations, exacerbating Obama's innate caution.

Biden is a deeply prideful man, and he has never fully forgiven Obama for quietly backing former Secretary of State Hillary Clinton in the 2016 campaign, and Biden still believes he could have beaten former President Donald Trump that year if given the chance. Nor was Biden pleased when Obama told him that he should consider sitting out 2020, too, people in his circle have said.

Obama wanted Sunday to be about Biden, a celebration of his accomplishments -- and does not feel pressured to act hastily, according to a former White House official who speaks with the former president regularly.