Shortly after President Joe Biden announced his decision to end his campaign for a second term, he endorsed Vice President Kamala Harris as his successor to be the Democratic Party's presumptive nominee.

Yet, while numerous other Democrats quickly fell in line and similarly endorsed Harris, many other high-profile Democrats have not, and it remains far from certain that Harris will ultimately receive her party's nomination, as several other prominent names have also been floated, according to CBS News.

To be sure, Harris is the odds-on favorite to be the Democratic standard-bearer in November's election, but she will likely have to first prevail over any potential challengers in an as-yet-undetermined process to pick the party's replacement for Biden.

Possible challengers to Harris

CBS News reported that VP Harris hit the ground running on Sunday once President Biden stepped aside and has already been blessed with enough pledged support from Democratic primary delegates to secure the nomination at the party's convention in August.

She is not yet the official nominee, however, and the outlet revealed another dozen big-name Democratic officials across the country who could potentially challenge Harris for that special status in the weeks ahead.

That list included in no particular order California Gov. Gavin Newsom, Michigan Gov. Gretchen Whitmer, Illinois Gov. JB Pritzker, Kentucky Gov. Andy Beshear, Maryland Gov. Wes Moore, Pennsylvania Gov. Josh Shapiro, Colorado Gov. Jared Polis, North Carolina Gov. Roy Cooper, Sen. Joe Manchin (I-WV), Sen. Amy Klobuchar (D-MN), Sen. Raphael Warnock (D-GA), and Transportation Sec. Pete Buttigieg.

Of course, some of those individuals have already or are expected to throw their support behind Harris, so that list doubles as not just potential challengers but also possible running mates for the new prospective head of the Democratic ticket.

Obama has yet to endorse Harris

There was a flood of endorsements for VP Harris' sudden candidacy in the wake of President Biden dropping his re-election bid, but one highly influential Democratic figure has yet to do so, according to the New York Post -- former President Barack Obama.

Instead, Obama published a brief Medium post praising Biden for his purported accomplishments throughout his decades-long political career as a senator, vice president, and now president.

Interestingly enough, Obama made no mention of Harris whatsoever in his post, but rather wrote, "We will be navigating uncharted waters in the days ahead. But I have extraordinary confidence that the leaders of our party will be able to create a process from which an outstanding nominee emerges."

Prominent Democrats delaying, withholding endorsements of Harris

Fortune magazine similarly reported on Monday that several other high-profile Democratic leaders hadn't yet issued endorsements for VP Harris -- though some may have now done so since that report was initially published.

That included current and former congressional leaders like Senate Majority Leader Chuck Schumer (D-NY), House Minority Leader Hakeem Jeffries (D-NY), and former House Speaker Nancy Pelosi (D-CA), among others.

The most likely reason for the withheld or delayed endorsements of Harris is that, per Fortune and multiple other media outlets, those figures played prominent roles behind the scenes in convincing Biden to drop out of the race, and those same figures didn't want to be seen as "puppeteers" pulling the strings in choosing Harris as the presumptive nominee.

Yet, Harris nonetheless appears to have momentum on her side to become the Democratic nominee, regardless of whether those big names quickly endorse her or not, and it remains to be seen how Democratic voters react to the abrupt shift at the top of their party's ticket after being assured for months that Biden was fine and ready to serve for another four years.