The billionaire businessman, who has switched parties several times throughout his political career, endorsed George W. Bush’s 2004 reelection campaign, contributed to John McCain and even held a fundraiser for a House GOP member as recently as last year.

And that’s not all. Beneficiaries of his largesse include former Sen. Orrin Hatch (R-Utah), a vigorous Trump defender, and Senate Appropriations Chairman Richard Shelby of Alabama, who once chaired the Senate Banking Committee.

Then there’s former Sen. Scott Brown (R-Mass.). Bloomberg endorsed and held a fundraiser for Brown when he ran against Elizabeth Warren in 2012.

“As a Democratic candidate for president, he makes an excellent Republican donor,” quipped Pennsylvania Democratic Lt. Gov. John Fetterman, who has not yet endorsed anyone in the primary.

Though Bloomberg has also spent millions to support Democrats in recent years, some Democratic elected officials and party operatives remain bitter that he worked to defeat them up and down the ballot in the past.

Bloomberg has contributed to Sen. Susan Collins (R-Maine), and Independence USA, a super PAC he funded, spent money to elect Republicans such as Sen. Pat Toomey of Pennsylvania, 2012 House candidate Andrew Roraback in Connecticut, and former Reps. Mike Fitzpatrick of Pennsylvania and Bob Dold of Illinois.

New York Democrats are particularly angered by his previous donations to Republicans, including some in the not-so-distant past. While spending about $100 million to help Democrats flip the House and win other midterm races last year, Bloomberg simultaneously donated to then-Rep. Dan Donovan, a Staten Island Republican, and held a fundraiser in June for Rep. Peter King, a Republican who represents parts of Long Island.

Co-hosts were asked to collect $10,000 for King. Bloomberg also showered New York Republicans with money when he was mayor, which helped them keep control of the state Senate.

“The path to win the House ran through New York: There were seven flippable seats in the state, and he supported Republicans in two of them,” said Monica Klein, a former consultant for Liuba Grechen Shirley, King’s 2018 Democratic opponent. “To come down and say he wants to be the head of the Democratic Party — the hubris is unbelievable.”

Bloomberg’s aides are addressing the issue of his past donations by highlighting that he has backed Republicans who support his priorities, such as liberal gun policies. He has also attempted to blunt criticism about spending his fortune on his own presidential bid by pledging to donate $100 million to an anti-Trump digital campaign.

“As mayor and as the country’s leading gun safety champion, Mike has supported Republicans in the past, including Republicans who helped deliver aid to New York after 9/11 or who crossed the aisle to vote for gun safety in Congress,” Bloomberg spokesman Jason Schechter said..

Gun control groups aligned with Bloomberg have thrown their weight behind some Republican candidates as well, in some cases giving them much-needed bipartisan bona fides. Everytown for Gun Safety, a nonprofit founded by Bloomberg, endorsed Rep. Brian Fitzpatrick (R-Pa.) in a Philadelphia-area swing district in 2018.

In 2016, Bloomberg reported spending nearly $10 million to successfully help reelect Toomey in his race against Democrat Katie McGinty. Independence USA ran TV ads in Philadelphia’s collar counties that drew attention to his support of gun control legislation, helping him win over critical moderate voters.

“The fact that Bloomberg was willing to throw his money into it gave Toomey a talking point to appeal to suburban voters in Philly, and ultimately one of the reasons we lost is that he outperformed Donald Trump in the Philly suburbs,” said Mike Mikus, McGinty’s former campaign manager. “I’m certain his hand in giving Mitch McConnell a majority in the Senate will be remembered by a lot of Democratic voters.”

Bloomberg left the Republican Party in 2007 and registered as an independent. He voted for Barack Obama in 2008, an aide said, though he did not publicize it at the time. He wrote an op-ed in favor of Obama during his 2012 reelection campaign and campaigned for Hillary Clinton in 2016. In 2018, he changed his registration to Democrat.

Fetterman, who ran unsuccessfully in the 2016 Senate primary, said there is no appetite for the billionaire’s candidacy among Pennsylvania Democrats: “Absolutely none that I’ve encountered.”

Over the weekend, liberal activists circulated a clip of Bloomberg speaking at the 2004 Republican National Convention in support of Bush.

“I want to thank President Bush for supporting New York City and changing the Homeland Security funding formula and for leading the global war on terrorism,” he said at the convention. “The president deserves our support. We are here to support him. And I am here to support him.”

Rebecca Katz, a New York-based consultant to progressive candidates, said the video is important context to understand Bloomberg.

“Bloomberg’s presidential campaign is only telling one side of the story. For years, he helped the New York state GOP hold on to their Republican majority. And while he’d prefer Democrats remember his 2016 convention speech, he also spoke at the RNC for George W. Bush in 2004,” she said. “The only thing that’s been consistent about his party affiliation is that it has always been about benefiting Michael Bloomberg."

Sally Goldenberg contributed to this report.