There are no long memories in politics, where today’s enemy can quickly become tomorrow’s best friend.

Take for example former Vice President Joe Biden, who has in the last week happily accepted the endorsement of a host of former Democratic presidential primary rivals as he vies for his party’s nomination – despite many of them not having the kindest words to say about him when they were still in the race.

New Jersey Sen. Cory Booker on Monday became the latest former primary hopeful to back Biden – saying that the former vice president will “restore honor to the Oval Office and tackle our most pressing challenges” – even though less than a year ago he was one of Biden’s biggest critics.

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While on the stump and on the debate stage last year, Booker went after Biden over a slew of issues ranging from the former vice president’s opposition to legalizing marijuana to his past work in the Senate on crime bills that led to mass incarcerations among African-Americans. Booker was also among the first candidates to criticize Biden’s remarks about working with segregationists in the Senate.

"Mr. Vice President has said that since the 1970s, every crime bill, major and minor, has had his name on it,” Booker said during a debate last July. “And, sir, those are your words, not mine, and this is one of those instances where the house was set on fire and you claimed responsibility for those laws. And you can’t just now come out with a plan to put out that fire."

Booker’s Senate colleague, former Democratic primary hopeful Kamala Harris of California, has also backed Biden’s White House bid despite pouncing on him for his checkered past when it comes to race relations.

During a debate in June, Harris trained her attention on the former vice president – calling his comment about working with segregationists “very hurtful” – and went on to slam Biden for opposing school busing in the 1970s.

“There was a little girl in California who was a part of the second class to integrate her public schools and she was bused to school every day,” she said. “And that little girl was me.”

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And it’s not just the more progressive candidates that now back Biden who have gone after him in the past. The other more establishment Democratic candidates in the race – of which Biden is one – have also opened fire on the person they’re now endorsing.

Sen. Amy Klobuchar of Minnesota was mocking Biden before he even got into the race, calling him out last March for saying that he would be the "most progressive" candidate in the Democratic field.

"He has been running things for a long time as a senator and as vice president, I'm sure he'll be able to point to some major accomplishments that are progressive," Klobuchar told CNN. "And then he'll have to explain things that weren't as progressive."

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Former South Bend, Ind., Mayor Pete Buttigieg, who Biden targeted as lacking experience for the White House job, hit back at the former vice president by saying all he does is use “the same Washington playbook and recycle the same arguments.”

“I hear Vice President Biden say that this is no time to take a risk on someone new,” Buttigieg said at a campaign event in Iowa in January, according to Time. “But history has shown us that the biggest risk we could take with a very important election coming up is to look to the same Washington playbook and recycle the same arguments.”

Then there is Michael Bloomberg, the billionaire former New York City mayor whose short presidential run ended a day after Super Tuesday and who quickly afterward endorsed Biden.

Rewind only a few months and Bloomberg was telling a different story about Biden – and the other Democrats in the primary battle – arguing that he was entering the fray because he didn’t see any candidates who could beat President Trump in November’s general election.

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That did not sit well with Biden, who snapped back at Bloomberg for a campaign video that suggested the former vice president supported Bloomberg's presidential bid – telling the Washington Examiner “I don’t endorse Republicans.” Bloomberg has been previously a registered Republican, and independent and a Democrat.

Biden also didn’t like it when Bloomberg ran a television ad seeking to tie himself to former President Obama.

Biden, who has repeatedly underscored his time serving as Obama’s vice president, said in a tweet that he would bring up Bloomberg’s past criticism of Obama – especially over the Affordable Care Act – in a debate.

“Welcome to the debates, Mike. We have a lot to catch up on about Barack Obama’s record,” Biden tweeted.