Californian state governor Gavin Newsom signed the law - the first of its kind in the US – on Tuesday last week.

It requires presidential candidates to disclose their tax returns in order to stand in the primaries. But the Republican Party has said this goes against the US constitution.

The law means a candidate who has not published their tax returns will be not appear on a primary election ballot, in a bid to “shed light” on, among other things, potential conflicts of interest.

During the 2016 campaign Donald Trump was the first presidential candidate since Gerald Ford in 1976 to not release his returns.

He has since repeatedly stated he will not do so during the 2020 campaign either.

In his lawsuit, he argued that together the first and fourteenth amendments to the constitution prohibit states taking actions that “affect a candidate’s first amendment rights”.

The lawsuit alleged: “The Democratic Party is on a crusade to obtain the President’s federal tax returns in the hopes of finding something they can use to harm him politically.

“In their rush to join this crusade, California Democrats have run afoul of these restrictions on state power over federal elections.”

Campaign group Judicial Watch has also sued California, warning of a “slippery slope”.

When signing the legislation, Newsom said: “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.

“The disclosure required by this bill will shed light on conflicts of interest, self-dealing, or influence from domestic and foreign business interest.”

He also published several comments from “nationally recognised leaders in constitutional law” who supported the move.