Senator Rand Paul promised to “take our country back” as he officially announced his bid for the Republican presidential nomination.Mr Paul, the son of former Congressman Ron Paul, is a favourite among Tea Party conservatives and libertarians.
The first-term senator and former eye surgeon has built a reputation on his willingness to challenge Republican orthodoxies.
Speaking at a rally in Louisville, Kentucky, Mr Paul chastised “career politicians” in Washington including members of his own party.
“Too often, Republicans have squandered victories by becoming part of the Washington machine,” he said.
“That’s not who I am.”
Mr Paul is the second fully declared candidate to seek the GOP nomination after Senator Ted Cruz of Texas, but he could eventually face as many as 20 rivals.
“I am running for president to return our country to the principles of liberty and limited government,”
Mr Paul has questioned the size of the US military and proposed relaxation of some drug laws that imprison offenders at a high cost to taxpayers.
He also challenges Republican support for surveillance programmes, drone policies and sanctions on Iran and Cuba.
On Tuesday, he vowed to end the government’s “unconstitutional” surveillance of Americans’ phone records if elected President.
As he seeks the nomination, Mr Paul is likely to reach out to more traditional voters as well.
The 52-year-old is tech savvy and youth-focused. He is a frequent poster on Twitter and, after his speech in Louisville, he answered questions from voters on his Facebook page.
His father, who enjoyed a large following among libertarian voters, ran and lost White House bids in 2008 and 2012.
Mr Paul starts the campaign in the second tier of Republican candidates, drawing the support of 8.4% of Republicans, according to a March Reuters/Ipsos tracking poll.
That puts him behind former Florida Governor Jeb Bush, who is considered a top contender among Republicans although he has not officially declared himself a presidential candidate.
Wisconsin Governor Scott Walker and former Arkansas Governor Mike Huckabee also have the edge on Mr Paul based on the March poll.
Mr Paul is statistically tied with Mr Cruz, New Jersey Governor Chris Christie, former neurosurgeon Ben Carson and Senator Marco Rubio of Florida.
Mr Rubio is expected to officially announce his bid for the presidency at an event in Miami on 13 April.