WASHINGTON — Congress is set to make it easier for political parties to collect bigger checks from their wealthiest backers — if Democrats do not scuttle the plan first.

Under a spending bill introduced in the Republican-led House, each superrich donor would be allowed to give almost $1.6 million per election cycle to political parties and their campaign committees. The comparable limit for 2014’s elections was $194,400.

The campaign finance proposal was tucked into an unrelated measure to keep most of the government open through the coming September.

The effort is the latest bid to weaken campaign finance rules passed after the Watergate scandal in the 1970s and updated a decade ago. It follows three Supreme Court rulings that gave rise to free-spending super PACs, which can accept unlimited contributions from people and corporations alike, and an increased role for outside groups to shape the outcome of elections.

House Democratic leader Nancy Pelosi joined others in her party in roundly criticizing the bill and said the campaign finance proposal needed to be ditched.

“The package includes a provision that would work to drown out the voices of the American people and massively expand the role of big money in our elections,” the California Democrat said in a statement.

She said the provision would increase the amount national political parties could collect 10-fold — and ignored the fact both parties had representatives at the table when the measure was drafted.

Individual donors to the Democratic National Committee or Republican National Committee this year were limited to $32,400. Under the new rules, individuals could annually give up to $324,000 to the national parties’ accounts for day-to-day operations, legal fights, nominating conventions and building improvements.

Their House and Senate campaign committees each could raise another $226,800 per donor each year.

Married couples could reach $3.1 million per election under the legislation.

The proposal has potential for both the Republican and Democratic parties but does nothing to restrict the cash that well-heeled donors can give elsewhere. But the higher limits might make donors more seriously consider party committees, which can coordinate with candidates, over super PACs, which cannot and sometimes are at odds with party strategy.

Watchdog groups decried the move as corrupting, cynical, out-of-touch and selfish.

“This backroom deal represents everything Americans detest about Washington and about Congress,” said Meredith McGehee, policy director at the Campaign Legal Center.

Added Lawrence Norden of New York University’s School of Law’s Brennan Center for Justice: “This raises the suspicion that politicians are trying to manipulate the campaign finance system for their own benefit, which will serve only to decrease average citizens’ participation in our electoral process.”

Another campaign finance activist, Democracy 21 President Fred Wertheimer, said “Republican and Democratic congressional leaders have chosen to embrace huge amounts of corrupting money for politicians over the interests and needs of the American people.”

The proposed campaign finance provision appears on page 1599 of the 1603-page bill that would keep most of the government open through September. The one exception is the Department of Homeland Security, which would run out of money at the end of February.

At the Capitol, House Speaker John Boehner said the agreement had backing from parties’ budget negotiators and defended the changes to campaign finance rules.

“The Congress is very concerned about taxpayer funding of political activities,” Boehner told reporters, reminding them that President Barack Obama earlier this year signed legislation that ended tax dollars to pay for parties’ nominating conventions.

That bill’s main focus was research on childhood illness but it effectively added about $20 million to each central party’s convention committee. Leaders from both parties say the cost could be as high as $60 million for each 2016 convention.

Following a closed-door caucus meeting, House Democratic leaders complained about parts of the bill, especially the changes to the political donation limits.

“I’m not very happy about it,” said Rep. Steny Hoyer, the Maryland Democrat tasked with rounding up votes for his party.