The decision on Thursday comes as Trump's Republicans are locked in a tight battle to keep control of both houses of the US Congress in the November midterm elections.
Trump has for years pushed the false claim that his 2020 election defeat was the result of widespread voter fraud and has criticised voting by mail.
The executive order signed by Trump on March 31 directed his administration to compile a list of confirmed US 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 required the US Postal Service to only deliver ballots to voters on each state's approved mail-in ballot list, and required states to preserve election-related records for five years.
In rejecting a request by plaintiffs including Senate Minority Leader Chuck Schumer of New York that he issue a preliminary injunction blocking the measure, Washington-based US District Judge Carl Nichols wrote that the Democrats had brought the case too early because the government had not yet produced any flawed citizenship lists and the Postal Service had not yet implemented any new rules.
"Given that the executive order does not command plaintiffs to do anything, and that no agency has yet acted pursuant to the Order in a way that could harm plaintiffs, they have not suffered any harm at present," wrote Nichols, who was appointed by Trump during his first term.
The judge said the Democrats could ask for an injunction again after federal agencies took steps to implement the executive order.
Democrats had argued the order infringed on individual states' rights to regulate elections under the US Constitution.
A coalition of Democratic states brought a similar lawsuit challenging the executive order in federal court in Boston.
US District Judge Indira Talwani, an appointee of Democratic former President Barack Obama, is due to hear arguments in that case on June 2.
