Indiana Republicans have defied intense pressure from President Donald Trump by rejecting his demands that they pass a voting map meant to favour their party in next year's midterm elections.

In one of the most conservative states in the US, 21 Republicans in the Senate joined all 10 Democrats to torpedo the redistricting plan by a vote of 31-19. The new map passed the House last week.

If it had cleared the legislature, Republicans could have flipped the only two Democratic-held congressional seats in the state.

Trump's call for Republican state leaders to redraw maps and help the party keep its congressional majority in Washington next year has triggered gerrymandering battles nationwide.

Republican-led Texas and Democratic-led California, two of the country's largest states, have led the charge. Other states where redistricting efforts have been initiated or passed include Utah, Ohio, New Hampshire, Missouri and Illinois.

Republican state Senator Spencer Deery said ahead of Thursday's vote: My opposition to mid-cycle gerrymandering is not in contrast to my conservative principles, my opposition is driven by them. As long as I have breath, I will use my voice to resist a federal government that attempts to bully, direct, and control this state or any state. Giving the federal government more power is not conservative.

Indiana Governor Mike Braun, a Republican, expressed his disappointment in the outcome, stating, I will be working with the President to challenge these people who do not represent the best interests of Hoosiers.

The revolt of Indiana Republicans came after direct months of lobbying from the White House. On Wednesday, Trump warned on his social media platform Truth Social that Republicans who did not support the initiative could risk losing their seats.

He directly addressed the Republican leader of the state Senate, Rodric Bray, calling him the only person in the United States of America who is against Republicans picking up extra seats.

For liberals, the outcome was a moment of celebration. Indianapolis-based activist Keith Wildstyle Paschall described the mood as jubilant, expressing relief that they wouldn't have to resort to legal strategies to defeat the redistricting directly at the statehouse.

Trump has hosted Indiana lawmakers at the White House in attempts to win over holdouts and dispatched Vice-President JD Vance to the state twice to shore up support. A number of Indiana Republican lawmakers reported being targeted with threats over the planned vote.

The rejection of the redistricting plan marks yet another setback for Trump, who has faced challenges in several states amid recent Democratic victories in off-year elections. Former White House chief strategist Steve Bannon stated that the party faces a huge problem and emphasized the urgency for Republicans to make gains in the upcoming redistricting battle.