A federal judge in Washington has refused to block enforcement of a ban on political contributions by individual government contractors.

U.S. District Judge James E. Boasberg declined to issue a preliminary injunction against the ban, saying he believed the contractors were not likely to win if the case went to trial.

Three people sued in October, charging that the ban on contributions violated their First Amendment rights and the equal protection guarantee of the Fifth Amendment. The law has been on the books for 70 years. It applies to individuals holding federal contracts, not companies.

In his April 16 ruling, Judge Boasberg said the ban “guards against ‘pay-to-play’ arrangements” and protects contractors from pressures to contribute.

“It is well established that preventing corruption or its appearance is a sufficiently important government interest to justify certain restrictions on political giving,” he wrote.

The plaintiffs argued that the ban is discriminatory because federal employees are not prohibited from making political donations. Judge Boasberg said the two groups are different because a federal contract can be worth more than a job.

Those filing suit were Wendy E. Wagner, a University of Texas law professor and a contractor for the Administrative Conference of the United States; Lawrence M.E. Brown, a human resources adviser to the U.S. Agency for International Development; and Jan W. Miller, a consultant to USAID.

They were represented by the American Civil Liberties Union. Their attorneys argued that there is no evidence that contractors are corrupting the political system. The judge said that “could simply be an indication that the ban is working.”