By Jan Wolfe

WASHINGTON, May 14 (Reuters) - U.S. President Donald Trump's executive order tightening rules on mail‑in voting faces its first major court ‌test on Thursday when a federal judge will hear arguments ‌by Democratic Party lawyers that it is unconstitutional and would disenfranchise millions of eligible voters.

A court ​hearing in Washington is set for 2 p.m. ET (1800 GMT) in a lawsuit brought by Democratic Party leaders seeking to block the March 31 executive order.

The case was brought by plaintiffs including Senate Minority Leader Chuck Schumer, House Minority ‌Leader Hakeem Jeffries, and ⁠the Democratic National Committee.

The litigation has been assigned to U.S. District Judge Carl Nichols, a Trump appointee, who is likely ⁠to issue a written decision at a later date.

Trump, a Republican, has for years pushed the false claim that his 2020 election defeat was the result of ​widespread ​voter fraud and has called for tighter ​rules on voting by mail ‌ahead of the November midterm elections, when his party will be trying to defend its narrow majorities in Congress.

His executive order directs his administration to compile a list of confirmed U.S. citizens eligible to vote in each state and to use federal data to help state election officials verify who ‌is eligible to vote.

It also requires the ​U.S. Postal Service to only deliver ballots to voters ​on each state's approved mail-in ​ballot list. States must also preserve election-related records for five ‌years.

The case being argued on Thursday ​has been consolidated ​with two other lawsuits challenging Trump's executive order brought by voting rights groups.

A similar lawsuit brought by a coalition of Democratic state attorneys is ​pending before a federal ‌judge in Boston.

The Justice Department has argued that the lawsuits are "premature" ​because federal agencies have not yet implemented Trump's executive order.

(Reporting by ​Jan Wolfe; Editing by Andrea Ricci )