WASHINGTON (REUTERS) - US Republican presidential candidate Rand Paul suspended his 2016 campaign on Wednesday (Feb 3) after his small-government campaign failed to gain traction with voters.

The senator from Kentucky was the second Republican candidate, behind former Arkansas Governor Mike Huckabee, to drop out of the race since Monday's night's Iowa caucuses.

That contest launched the parties' process to nominate candidates for the November election.

"It's been an incredible honour to run a principled campaign for the White House. Today, I will end where I began, ready and willing to fight for the cause of Liberty," Mr Paul said in a statement.

In the Republican race, the libertarian-leaning Paul finished in fifth place in Iowa with 4.5 per cent of the vote. US Senator Ted Cruz finished first in Iowa, putting a dent in real estate tycoon Donald Trump's standing as front-runner to be his party's nominee.

Paul, who is also focused on winning another US Senate term, said he would continue fighting for limited government, criminal justice reform and "reasonable" foreign policy.

The heir apparent to the libertarian-minded voters who helped his father gain a standing in the last two presidential elections, Paul struggled to attract support in a crowded Republican field. His withdrawal leaves 10 Republican candidates in the 2016 White House race.