Rand Paul joins 2016 presidential campaign in US

Sen Rand Paul, a favourite of the ultraconservative tea party movement and frequent antagonist of Republican Party leaders, declared his candidacy for president.

Paul began his White House campaign today, kicking off the presidential run with a rally in downtown Louisville, and will be jetting to early nominating states with a pitch aimed at the libertarian wing of the Republican Party.

Paul begins the 2016 race as the second fully declared candidate, behind Sen Ted Cruz of Texas, but he could face as many as 20 rivals for the nomination before the lead-off Iowa caucuses in January.

Along the way, the first-term senator from Kentucky is likely to challenge his fellow Republicans' views on both foreign and domestic policy, as well as the nuts and bolts of how campaigns are run. Tech savvy and youth-focused, Paul is expected to be an Internet juggernaut his competitors will be forced to chase.

After his speech in Louisville, Paul was set to answer questions from voters on his Facebook page.

On the eve of his launch, Paul was a frequent poster on Twitter.

"On April 7, one leader will stand up to defeat the Washington machine and unleash the American dream," Paul's political committee announced in a Web video before Tuesday's event.

It's unclear how much support Paul can muster in the Republican mainstream. His father, former US Rep Ron Paul of Texas, unsuccessfully sought the Republican nomination several times, appealing to libertarians who favour limited government and oppose tax increases.

Paul is a frequent contrarian against his party's orthodoxy, questioning the size of the US military and proposing relaxation of some drug laws that imprison offenders at a high cost to taxpayers. He also challenges Republicans' support for surveillance programs, drone policies and sanctions on Iran and Cuba.