Sen. Joe Lieberman (I-Conn.) will host a fundraiser for Senate Majority Leader Harry ReidHarry Mason ReidWATCH: There is no Trump-Russia collusion and the media should stop pushing this The demise of debate in Congress ‘North by Northwest,’ the Carter Page remake MORE (D-Nev.), whom he has often antagonized, on Sunday in Greenwich, Conn.

Lieberman left the Democratic Party after losing his Senate primary in 2006 and has had a tempestuous relationship with Democrats ever since.

He caucuses with his former party but he campaigned for Republican Sen. John McCainJohn Sidney McCainLawmakers worry about rise of fake video technology Democrats put Dreamers and their party in danger by playing hardball Trump set a good defense budget, but here is how to make it better MORE (Ariz.) in the 2008 presidential campaign, and many Democrats were unhappy with his criticism of Barack ObamaBarack Hussein ObamaOvernight Energy: Dems ask Pruitt to justify first-class travel | Obama EPA chief says reg rollback won't stand | Ex-adviser expects Trump to eventually rejoin Paris accord Overnight Regulation: Trump to take steps to ban bump stocks | Trump eases rules on insurance sold outside of ObamaCare | FCC to officially rescind net neutrality Thursday | Obama EPA chief: Reg rollback won't stand Ex-US ambassador: Mueller is the one who is tough on Russia MORE.

Reid was under pressure in January 2009 to yank Lieberman’s chairmanship of the Homeland Security Committee, but decided against it.

Lieberman spokeswoman Erika Masonhall said the two men remain close.

"Sen. Lieberman supports his good friend Sen. Reid and was more than happy to help in his reelection efforts," Masonhall said.

From the Connecticut Post:

The gesture of goodwill toward Reid, who was quoted earlier this year as saying that Lieberman double-crossed him on health care reform, has thrown even the most politically astute for a loop.

Connecticut's top Democrat had to go back more than a year -- to a fundraiser coinciding with President Barack Obama's inauguration -- to recall the last time she saw Lieberman at a party event.

"If they're friends, he could be helping a friend. I don't know, though," said Nancy DiNardo, the state Democratic Party chairwoman.

Democrats in the know say this is the second time in about a year that Reid has hit the Greenwich fundraising circuit, taking advantage of the deep pockets of a town elite that doesn't discriminate when it comes to political stripe.

Hosting Reid this time around is venture capitalist Dan Ciporin and his wife, Jill, at their $4 million midcountry home, according to the reception's invitation, which suggests a minimum donation of $1,000 per person. Individuals can donate up to $2,400 for the primary and $2,400 for the general election to Reid, who is seeking a fifth term.