Texas Senator Ted Cruz scored a decisive victory over billionaire businessman Donald Trump in the Kansas caucuses Saturday, giving the conservative lawmaker a boost in efforts to unseat the current Republican front-runner.
Cruz led with 51 per cent of ballots to Trump's 24 per cent, CNN reported with some 54 per cent of ballots counted. Florida Senator Marco Rubio had 14 per cent and Ohio Governor John Kasich had 9 per cent.
Cruz was also leading in caucuses in the north-eastern state of Maine.
Voting was underway Saturday as five US states went to the polls to make their choices for who should square off in November's presidential election.
The elections are the first since Super Tuesday, in which a dozen states made their decisions and solidified the front-runner status of both Trump and Democrat Hillary Clinton.
Both Democrats and Republicans vote in Kansas and Louisiana, while Republicans caucus in Kentucky and Maine, and Democrats hold similar meetings in Nebraska.
The Republican establishment has been struggling to prevent newcomer Trump from seizing the conservative party's nomination for the general election.
Cruz also received a psychological boost from a closely watched straw poll of conservative activists meeting outside Washington who named him their pick for president.
Nevertheless, Trump remains well ahead in polls for the Republican contests, though he has yet to amass enough delegates to put him entirely out of reach of his competitors.
This week both Mitt Romney and John McCain, the 2012 and 2008 Republican presidential nominees respectively, spoke forcefully against Trump being the nominee.
In the Democratic races, Clinton leads voter surveys over Senator Bernie Sanders of Vermont.
Trump has 329 delegates so far, compared to his nearest rival, Cruz, with 231, according to RealClearPolitics; 1,237 delegates are needed to win.
Clinton has won 1,058 delegates - almost half the amount needed - while Sanders has 431 after Super Tuesday.