Michael Bloomberg, the billionaire former mayor of New York City, has decided to throw his political clout and personal fortune behind the Democratic campaign to take control of the House of Representatives this year, directing aides to spend tens of millions of dollars in an effort to expel Republicans from power.

Bloomberg — a political independent who has championed left-of-center policies on gun control, immigration and the environment — has approved a plan to pour at least $80 million into the 2018 election, with the bulk of that money going to support Democratic congressional candidates, advisers to Bloomberg said.

By siding so emphatically with one party, Bloomberg has the potential to upend the financial dynamics of the midterm campaign, which have appeared to favor Republicans up to this point. Facing intense opposition to President Donald Trump and conservative policies, Republicans have been counting on a strong economy and heavily funded outside groups to give them a political advantage in key races, especially in affluent suburbs where it is expensive to run television ads.

Bloomberg’s intervention is likely to undermine that financial advantage by bankrolling advertising on television, online and in the mail for Democratic candidates in a dozen or more congressional districts, chiefly in moderate suburban areas where Trump is unpopular. Democrats need to gain 23 congressional seats to win a majority.

While Bloomberg has not chosen his list of targeted races yet, he is unlikely to get involved in rural, conservative-leaning districts where his views on guns and other issues could stir an uproar, according to people briefed on his plans, some of whom spoke on condition of anonymity to discuss private deliberations.

The new alliance between Bloomberg, 76, and congressional Democrats marks a fresh stage in the former mayor’s political evolution. After moving freely between elite circles in both parties for years, Bloomberg is now poised to become one of the Democrats’ most important benefactors: His spending on House campaigns appears likely to exceed the involvement of donors like Sheldon Adelson, the Republican casino billionaire who recently donated $30 million to a super PAC aligned with House Speaker Paul Ryan, and Tom Steyer, the liberal hedge-fund investor spending tens of millions of dollars on voter-turnout programs and television ads demanding Trump’s impeachment.

Bloomberg outlined his plans in a statement, denouncing the Republican Congress and urging a return to divided control of the government.

Calling Republican leaders in the House “absolutely feckless,” Bloomberg criticized them for failing to check Trump or to exercise rigorous oversight of his presidential administration.

“I’ve never thought that the public is well-served when one party is entirely out of power, and I think the past year and half has been evidence of that,” Bloomberg said, lamenting that Republicans “have done little to reach across the aisle to craft bipartisan solutions — not only on guns and climate change, but also on jobs, immigration, health care and infrastructure.”

Bloomberg continued: “Republicans in Congress have had almost two years to prove they could govern responsibly. They failed.”

Bloomberg will likely support Republicans in a few races for governor this year, and he donated $5,400 in April to Rep. Dan Donovan, a Republican battling a primary challenge in the New York City borough of Staten Island from Michael Grimm, a former congressman who was jailed for tax evasion. Beyond that, Bloomberg is expected to spend little or nothing on Republicans at the federal level, his advisers said. And Bloomberg, despite his antipathy for the GOP, has not shed some of his reservations about the Democrats. He has indicated to aides that he wants to support only candidates who share his relatively moderate political orientation, avoiding nominees hailing from the populist left. In his statement, Bloomberg also took strong issue with any Democrats campaigning on impeachment, declaring: “Nothing could be more irresponsible.”