The Texas Democratic Party is suing to block a state law that will end straight-ticket voting, starting with the November general election.
The party filed the federal lawsuit on Thursday in Laredo.
The lawsuit claims the new law will make long lines worse at polling places, and will disproportionately affect black and Hispanic voters who tend to live in cities with long ballots.
Harris County has the longest ballot in the nation.
The Texas Legislature voted in 2017 to ban straight-ticket voting, with most Republicans voting to end the practice, and most Democrats opposed to the change.
Historically, 67% of Texans vote straight party.