US President Donald Trump's push to redraw congressional borders in Republicans' favor before November's midterm elections suffered twin setbacks Tuesday, as South Carolina lawmakers blocked a new map and a court halted Alabama's redistricting plan.

The blows slowed a broader Republican effort, urged by Trump, to reshape voting districts in conservative-led states as the party tries to protect its narrow majority in the US House of Representatives.

In South Carolina, a bloc of Republican state senators joined Democrats to stop a last-minute plan that would likely have handed them control of all seven of the state's House seats.

The proposal would have targeted the district of Congressman James Clyburn, the lone Democrat in South Carolina's delegation and one of the most influential Black lawmakers in US history.

The state House had already approved new district lines, but the Senate rejected the blueprint after early voting began Tuesday for South Carolina's scheduled June primary.

"Neither my conscience nor common sense will allow me to stop an election that has already begun," Republican state Senator Richard Cash said in a statement carried by US media.

The move effectively blocks the new map ahead of the November midterms, though Republicans could revive the effort in a future session.

Hours earlier, a three-judge federal panel blocked Alabama from using a Republican-drawn map that would have given the party an edge in six of the state's seven congressional districts.

The court said Alabama's plan intentionally discriminated against Black voters by spreading them across districts "to dilute votes, at least in part because they are Black."