New York City’s fiscal watchdog — a potential 2017 rival to Mayor Bill de Blasio — Thursday labeled a nonprofit linked to the mayor and able to raise unlimited cash a “slush fund” and said strict rules should be enacted to curb the use of such organizations.

City Comptroller Scott Stringer criticized fellow Democrat de Blasio for the mayor’s now-defunct Campaign for One New York, which was established to advance his agenda including universal prekindergarten.

“I have never felt that the way to govern is to set up a slush fund and think it’s in the best interest of the city,” Stringer said at an unrelated news conference in lower Manhattan.

De Blasio, at a Bronx event later Thursday, said Stringer “doesn’t know what he’s talking about.”

The Campaign for One New York “fully disclosed” its fundraising and spending — doing so voluntarily — and played by the rules, the mayor said.

Voluntary disclosures show the lobbying group has raised $4.4 million in 2014 and 2015 from donors such as real estate developers who had or wanted business before the city.

Rose Gill Hearn, the chairwoman of the city’s Campaign Finance Board, criticized the nonprofit’s ability to raise unlimited funds even after de Blasio took office, during an agency meeting Wednesday.

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Though the agency dismissed an ethics complaint against the group, Hearn said fundraising limits that work during a campaign shouldn’t be discarded once a candidate is elected.

Good-government groups, including Manhattan-based Citizens Union, want a city law requiring fundraising nonprofits be regulated by the finance board.

Under the U.S. Supreme Court’s 2010 Citizens United decision, entities like de Blasio’s nonprofit can freely raise and spend money and are not bound to the same stringent rules as traditional political campaigns.

The mayor has said he does not support the decision in principle and repeated Thursday that he wants all groups — including those who oppose or favor him — to lose the power to raise unlimited funds.

Stringer said Thursday that Gill Hearn’s criticism “is appropriate and I think it’s also a warning to elected officials that you should stay and play by the laws.”

He discounted de Blasio’s consistent defense of the group’s practices as by the books.

“The standard should not be whether you broke the law,” Stringer said. “The standard should be what’s the best practice for conducting political campaigns.”

Stringer is considered a potential challenger to de Blasio’s expected run for re-election.

Wednesday night, he held a happy hour fundraiser for an undeclared office. As of Jan. 11, Stringer had raised $1,169,104 for a possible 2017 campaign, again, for an undeclared office, according to filings with the Campaign Finance Board.

De Blasio had $890,657 for use on his re-election campaign, the filings showed.