He then argued that his vote was based on a belief that Biden is the strongest candidate to defeat Trump: "There is a reason Trump fears @JoeBiden and roots for Bernie. #Biden2020.”

But that endorsement was met with a chilly reception by at least one Biden staffer.

“Yes, customer service? I just received a package that I very much did not order. How can I return it, free of charge?” Andrew Bates, a communications staffer for Biden and an alum of Hillary Clinton's 2016 campaign, retorted in a tweet, later joking in a second post that his dig was meant as a "lighthearted joke."

Comey served as deputy attorney general in George W. Bush’s Justice Department before Obama tapped him to lead the FBI. He was a lifelong Republican before revealing in the summer before the 2016 election that he’d ditched his party affiliation.

The former FBI director has become a reviled character within the GOP, lead by Trump's accusations that Comey is a treasonous, crooked cop who improperly investigated his 2016 campaign. Clinton and those who worked for her campaign have also continued to hold a grudge against Comey, blaming his 11th-hour reopening of the bureau's probe into the 2016 nominee's email practices during her tenure as secretary of state for her narrow loss.

But the former FBI director has also become a cult hero of sorts for opponents of the president: It was Comey's firing that triggered the appointment of special counsel Robert Mueller to investigate allegations of collusion between the Russian government and Trump's 2016 campaign.