A supporter of progressive priorities like gun control and climate change, Mr. Bloomberg has also given millions to Republicans he felt shared his goals, irritating some Democrats.

Michael R. Bloomberg during a campaign stop in Norfolk, Va., on Monday.Credit…Sarah Silbiger for The New York Times

By Jeremy W. Peters and Stephanie Saul

WASHINGTON — In 2016, Democrats thought they had found the perfect candidate to win a United States Senate seat in Pennsylvania and put them within striking distance of taking back the majority. But Katie McGinty, an environmental policy expert with degrees in chemistry and law, ran into an overwhelming obstacle: Michael R. Bloomberg’s fortune.

The former mayor of New York poured in $11.7 million to help re-elect the Republican incumbent, Senator Pat Toomey, who had led an effort, albeit unsuccessful, to expand background checks for gun purchasers, a top priority of Mr. Bloomberg’s.

Mr. Toomey won by less than two percentage points, handing a key victory to the Senate majority leader, Mitch McConnell: The Republicans held on to control of the chamber by two seats. At the time, it was the most expensive Senate race the country had ever seen, and Mr. Bloomberg’s money was one of the largest influxes of outside influence.

As Mr. Bloomberg begins his campaign for the White House with a promise to spend as much as it takes to defeat President Trump, his Democratic rivals are accusing him of trying to buy his way into the Oval Office. But his political spending on behalf of Republicans is also coming under attack from Democrats who say that his overlapping political goals with Republicans in Washington, and in particular with Mr. McConnell, are disqualifying.

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“For many he went too far when he gave money to Pat Toomey,” said Jim Manley, a Democratic consultant and former senior aide to Harry Reid, the previous Senate leader, pointing out that Ms. McGinty also favored stricter gun regulations and that the race was so close.

“He tries to play both sides, and he ends up burning Democrats,” Mr. Manley added. “If that makes you feel good, I’m glad. But the reality is there are no free shots.”

Federal records show that political committees funded by Mr. Bloomberg have spent more than $86 million since 2012 — the bulk devoted to promoting Democrats. Yet more than $17 million went to boost Republicans. In addition, Mr. Bloomberg has personally donated another $950,000 to Republican campaigns and political action committees.

A review of Mr. Bloomberg’s giving shows that he has not only backed Republicans in competitive and pivotal races like Mr. Toomey’s, but that he has also sunk money into Republican primaries on behalf of McConnell allies. They include Senator Lindsey Graham of South Carolina — Mr. Bloomberg helped Mr. Graham in 2014 by giving $250,000 to a PAC supporting him — and the late Senator Thad Cochran of Mississippi, who in 2014 fought off a Tea Party insurgent by just a few thousand votes and had $250,000 in support from Mr. Bloomberg.

Mr. Bloomberg, who has been a Democrat, a Republican and an independent over the last two decades, has taken by far the most unusual approach to political donations of any Democrat now running for the 2020 nomination. With huge resources, he has tended to back candidates who share his views on priorities like gun control and climate change.

In 2014, as several vulnerable Democratic senators in conservative leaning states were defeated — and Republicans took control of the Senate for the first time since 2006 — some party leaders believed Mr. Bloomberg made an inhospitable political climate even worse. He wrote a letter to some of the wealthiest contributors in the Democratic Party in New York, urging them not to donate to four Democrats who voted to block background check legislation. And his donations helped pay for ads that attacked them.

A Mayors Against Illegal Guns ad from 2013, funded by Mr. Bloomberg, pushing senators in key states, including Senator Heidi Heitkamp of North Dakota.

Three of the four — Mark Pryor of Arkansas, Mark Begich of Alaska and Heidi Heitkamp of North Dakota — eventually lost their seats. The fourth, Max Baucus of Montana, chose not to seek re-election and became President Barack Obama’s ambassador to China.

A statement issued by Mr. Bloomberg’s campaign Tuesday said that the former mayor worked with Republicans to “get things done,” citing his efforts to push for gun reform as well as financial support for the city following the Sept. 11, 2001, attacks on the World Trade Center and after Superstorm Sandy.

“He doesn’t share their values,” read the statement, referring to Republicans, “and spent approximately $100 million of his own money to help flip Congress to the Democrats in 2018 and, more recently, help flip the Virginia legislature in 2019. But after he asked people to buck their party, do the right thing and be on the right side of history, committees he’s supported have sometimes reflected that.”

Mr. Bloomberg’s supporters say his backing of Republicans is the kind of act that defines his approach to politics and makes him an appealing presidential candidate: contrarian, independent and unburdened about whether he will anger key allies. And donating to both parties is a longstanding practice for many elite donors and businessmen — including Mr. Trump, before he became president — who want to have the ear of whichever party is in power.

People on both sides of the aisle who have worked with Mr. Bloomberg’s political operation said that he was always strategic about his alliances on Capitol Hill. He was in constant contact with Republican and Democratic leaders, who tried to keep him close and sought out his help in key races even when his contributions were working against them elsewhere.

“They didn’t ever operate completely in a silo,” said Josh Holmes, Mr. McConnell’s former chief of staff. “They were talking with Republicans; they were talking with Democrats. And they wanted to win. I probably disagree with 90 percent of what they’re doing. But they’re extremely effective.”

For example, even as he made Mr. Toomey’s re-election a priority in 2016, his money — nearly $4.3 million — simultaneously went toward defeating Senator Kelly Ayotte in New Hampshire, a Republican who opposed the background check bill.

Mr. Bloomberg’s giving fits with his political background. He was a longtime Democrat when he switched his registration to run for mayor as a Republican in 2001. He left the Republican Party in 2007 and chose to be unaffiliated with either party until 2018 when he rejoined the Democratic Party.

The Toomey-McGinty race in Pennsylvania is the one that left the most bitter taste for many Democrats because they saw no path to retaking the Senate without winning that seat. Some said they found it frustrating that Mr. Bloomberg did not agree that it would be more helpful to his long term goals on gun control to have a Democrat in that seat and be one step closer to a Democratic-controlled Senate than it would be to see Mr. Toomey re-elected.

In an interview with The New York Times in 2013, Mr. Bloomberg explained his logic, saying that if Democrats could not get senators on board with something as popular with the public as background checks, they should pay a political price. “What I would suggest is that they have all of their members vote for things that the public wants,” he said. “And if they don’t do that, the voters should elect different senators who will listen to them.’’

Mr. Bloomberg at a gun safety rally for Moms Demand Action in New Hampshire last year.Credit…Cheryl Senter/Associated Press

Though it may have worked against individual Democratic candidates, Mr. Bloomberg’s tactic of punishing those who did not support gun control created a new dynamic in the Democratic Party. The National Rifle Association had long employed the same tactics, and Mr. Bloomberg and his aides believed they needed to create the same fear of retribution on their side.

Some Democrats said this was a laudable goal. “Don’t get me wrong, I do not support the idea of the Bloomberg candidacy,” said Brian Fallon, a former aide to Senator Chuck Schumer now running Demand Justice, a group that works to counter conservative influence in the federal courts. “But as someone who is now working on issue advocacy, I can sort of appreciate the approach he took to try to shame people into prioritizing an issue that was viewed for a long time as something Democrats should soft pedal for political reasons.”

By 2018, it appears, Mr. Bloomberg had decided that this strategy needed to be supplemented with broader electoral goals. He spent heavily on 24 Democratic congressional candidates in a successful effort to help deliver the House to Democrats. At the time, he defended his previous efforts to help Republicans he described as serious, “like my friend John McCain,” he wrote in an op-ed article. “But too many,” he added, “have been absolutely feckless.”

But his disillusion with the Republican Party is fairly recent. Over the years he has supported Republican candidates ranging from President George Bush in 1992 to Senator Scott Brown of Massachusetts in 2012 — the beneficiary of a fund-raiser at Mr. Bloomberg’s Manhattan townhouse that year.

Mr. Brown lost to Senator Elizabeth Warren, who now appears to be on a collision course with Mr. Bloomberg in the Democratic primary. He has said her proposal to hit billionaires like him with a wealth tax is probably unconstitutional.

Still, some Democrats view Mr. Bloomberg’s past support of Republicans as a potential drag on his campaign, but not a deal breaker.

“I think he’s going to have to answer, but I would hope that Democrats would be willing to accept anybody in our party these days who is willing to evolve,” said Lachlan McIntosh, a South Carolina political operative. “The country is in desperate shape. We’ve got to beat Trump.”

Rachel Shorey and Annie Daniel contributed reporting.

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