SACRAMENTO, Calif. (Reuters) - A conservative legal group has filed a lawsuit seeking to block a new California law aimed at forcing Republican U.S. President Donald Trump to release his tax returns, the latest volley over Democrats’ efforts to see the former businessman’s financial records.

U.S. President Donald Trump boards Air Force One at Morristown municipal airport en route to Washington after a weekend in Bedminster, New Jersey, U.S., August 4, 2019. REUTERS/Yuri Gripas

The lawsuit, filed in federal court in Sacramento on Aug. 1 by the group Judicial Watch, argues the statute is unconstitutional because it sets up illegal new rules governing who can seek the presidency.

The measure requires presidential candidates to release five years worth of tax returns in order to appear on a nominating ballot in California, the most populous U.S. state. The bill passed both houses of California’s Democrat-controlled legislature and was signed into law by Democratic Governor Gavin Newsom last week.

“It’s a bad law as applied to anyone, including President Trump,” said Tom Fitton, president of Washington, D.C.-based Judicial Watch. The lawsuit was filed on behalf of four voters registered in California - two Republicans, a Democrat and an independent.

Trump refused to release his tax returns during the 2016 a campaign, a practice that has been followed by every presidential nominee for decades.

Last month, the Democrat-controlled Ways and Means Committee of the U.S. House of Representatives sued the U.S. Treasury Department to force the release of Trump’s tax records. Democrats want the tax returns as part of their inquiry into possible conflicts of interest posed by Trump’s continued ownership of his extensive business interests.

In New York, Governor Andrew Cuomo, also a Democrat, signed an amendment last month to a law requiring the state’s Department of Taxation and Finance to release any returns sought by the congressional committees.

Both efforts have been rebuffed by Trump’s team. The president sued to block the New York law, and Treasury Secretary Steve Mnuchin has refused to hand Trump’s returns over to the Ways and Means Committee.

An earlier version of the California law had been vetoed by Newsom’s predecessor as governor, Democrat Jerry Brown, who expressed concerns over its constitutionality.

Newsom’s office declined to comment on the lawsuit on Monday, but directed Reuters to the governor’s statement when he announced the bill signing last week.

“These are extraordinary times and states have a legal and moral duty to do everything in their power to ensure leaders seeking the highest offices meet minimal standards, and to restore public confidence,” Newsom said in his statement.