ALBANY — Gov. Andrew M. Cuomo signed into law yesterday an ethics measure that he said would reduce the influence of big campaign donors.

But government watchdog groups had opposed the law because it focused more on nonprofits than on corrupt politicians. The critics also said the state law wouldn’t likely withstand a federal legal challenge.

Cuomo, a Democrat, said the new law would reduce the impact of a 2010 U.S. Supreme Court decision known as Citizens United, which helped wealthy donors dramatically increase campaign spending through the use of thinly veiled “independent expenditure” groups to tout or criticize a candidate. The state measure would toughen restrictions on such groups by prohibiting any coordination by them with the candidates they favor.

The new law represented a shift by the governor, who, in a 2015 interview, said that Citizens United could be addressed only by federal court or congressional actions, not states’. Now Cuomo said the new law would help limit the “sordid influence of dark money” in campaigns.

“These actions roll back the disastrous influence of Citizens United and prohibit coordination between candidates and independent expenditure committees,” Cuomo said in a statement. The governor himself was aided in 2011 and 2012 by a group called Committee to Save New York, which spent $17 million to advance his agenda.

Notably, the new law was opposed by the same watchdog groups that typically urge lawmakers to rein in campaign contribution limits.

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Common Cause, Citizens Union and the New York Public Interest Research Group contended the law was “astoundingly” broad, would place “onerous” reporting requirements on nonprofits, which are regulated tightly already, and took the focus off the root causes of major scandals that resulted in the convictions of two legislative leaders in the last nine months: lawmakers’ ability to receive unlimited outside income and companies’ ability to form an unlimited number of subsidiaries to skirt restrictions on corporate contributions.

“Common Cause/NY is deeply disappointed that Governor Andrew Cuomo has chosen to ignore the causes of systematic corruption in Albany, and instead wrongfully punish organizations’ proper and lawful collaboration to more effectively advocate for their causes,” said Susan Lerner, Common Cause’s executive director. “Any serious attempt by the Governor and the legislature to strengthen ethics laws must include a ban on outside income, an end to the LLC loophole, and greater transparency within the state’s budget process.”

She added the group “strongly believes that the new law will not stand up on appeal” because of campaign spending protections encoded in the Citizens United decision.