An upcoming book chronicling the relationship between Barack Obama and Joe Biden reveals that the 'bromance' the two grew to share during the White House years has fizzled in the post-White House years, according to "Barack and Joe: The Making of an Extraordinary Partnership."

Written by the Washington Post's nonfiction editor, Steven Levingston, Barack and Joe sheds light on trials and tribulations between the two very different politicians, which may explain Obama's refusal to endorse Biden's 2020 bid for the White House, according to the Daily Mail.

The obvious answer is that Obama worked with the guy for eight years and knows he's a gaffe-prone, gropey, hair-sniffing racist with political baggage - but feel free to continue reading.

Happy 55th, Barack! A brother to me, a best friend forever. pic.twitter.com/uNsxouTKOO — VP Biden (Archived) (@VP44) August 4, 2016

While the two men did develop a strong affection for one another, there were times Biden drove Obama over the edge as the book reveals a younger Obama once rolled his eyes at Biden's constant babbling, sending a note to his adviser saying: 'Shoot. Me. Now.' And it was a political embarrassment for Biden when Obama failed to step up and support his former vice president in both the 2016 and 2020 presidential races. -Daily Mail

In 2016, Joe was passed over in favor of Hillary Clinton - as Obama was reportedly more concerned about what a GOP victory would mean for his doomed health care program, among other things.

"Joe, despite his many virtues, was just another white guy, one in a long line of American presidents — hardly the symbol of the Teutonic change that Obama hoped would mark his place in the history books," writes Levingston.

"Barack had placed his bet on Hillary, the one he believed would confirm his revolutionary stamp on American's political culture - the first black president passing the baton to the first woman president."

As far as the 2020 election, Obama said in January 2019 that Democratic party leadership needed "new blood."

Biden, meanwhile, said he doesn't want Obama's endorsement - stating that "Whoever wins the nomination should win it on their own merits," after Obama declined to support him.

That said, Obama has met with other potential presidential candidates, including Texas Congressman Beto O'Rourke, prior to Biden's decision to run.

What had been an unprecedented closeness between a president and vice president had changed. 'Now they were two high profile politicians considering their own futures' — and no longer brothers in arms they once were. In the beginning, 'from a gaffe-meister, Biden had shaped himself into a conscientious, well informed partner to the president emerging as the heart to Obama's brain', write the authors. Biden always had his eye on the Oval Office and made a promise to his son, Beau, when he was dying of brain cancer in 2015, that he would try for a third run for the presidency despite his lousy track record. -Daily Mail

The rest of the Daily Mail's article - along with Levingston's book - rattles off Obama and Biden's on-again / off-again trials and tribulations, such as the time Biden said of Obama "I mean you got the first sort of mainstream African American who is articulate and bright and clean and a nice-looking guy," to which Obama noted that there had been prior black presidential candidates - all of whom were (arguably) articulate - Jesse Jackson, Shirley Chisholm and Al Sharpton.

When Obama picked Biden as his running mate, he said "I love this guy and he's got heart."

Now, Biden has nothing but problems - and there's no sign of Obama's aforementioned love for him.