DeLay, who is being prosecuted for allegedly violating state campaign finance law, ran for re-election and won the Republican primary earlier this year -- but then decided that his legal troubles were putting his seat at risk. He moved to Virginia and said he was withdrawing as a candidate in the 22nd district of Texas.
Democrats challenged state Republicans' contention that the party could choose a replacement candidate. U.S. District Judge Sam Sparks ruled Thursday that DeLay must remain on the GOP ballot for November's election.
Republican Party lawyer James Bopp Jr. said an appeal would be filed with the Fifth Circuit Court of Appeals, The New York Times reported.
Sparks, a Republican appointee, said last week that allowing parties to drop unpopular candidates could lead to abuse.
"It can happen in every race in this state for every office," Sparks said.