Sen. Rand Paul told supporters in combative speech that he was running for president because he wanted "a return to government restrained by the constitution" and "a return to privacy, opportunity, liberty."
"Too often when Republicans have won, we've squandered our victory by becoming part of the Washington machine," he said at a Louisville rally on Tuesday, as he officially kicked off his bid for his party's nomination for 2016. "That's not who I am."
Hours earlier, he had announced his plans on his website, saying he wanted to return the country to principles of liberty and limited government. To his cheering supporters on Tuesday, he said, "We have come to take our country back."
Paul, a tea party favorite and frequent antagonist of leaders of his Republican Party, is ready to test how much change voters want, both for their government and for the GOP.
"If we nominate a candidate who is simply Democrat light, what's the point?" he said. "Why bother?"
He said that he wanted to be part of a return to prosperity for all Americans. Hope and opportunity are slipping away, he said.
"It seems to me that both parties and the entire political system are to blame," he said.
National debt has increased under both Republican and Democratic administrations, threatening both the country's economy and its security, he said. The U.S. Congress will not balance the federal budget unless it is forced to by a constitutional amendment, he said.
"We can wake up now and do the right thing: Quit spending money we don’t have," he said.
Under both parties, the country's poor have gotten poorer and the rich have gotten richer, he said. He called for economic zones for such impoverished areas as Detroit and west Louisville and for dramatically lower taxes for companies that return profits to the United States.
As far as foreign policy, he said, "The enemy is radical Islam," he said. "You can't get around it."
The United States needs a robust national defense unemcumbered by nation building, he said. Any agreement over Iran's nuclear program must be approved by Congress, he said.
"Not only is that good policy, it's the law," he said.
After his Louisville rally, he will jet to early nominating states with a pitch aimed at the libertarian corners of the GOP. He will hold a rally in New Hampshire on Wednesday, his seventh visit in a year's time to the state that's home to the nation's first primary.
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 early next year.
Along the way, the first-term senator 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. His new website left no doubt about his plans, outlining more than a dozen positions on issues he would take if elected president.
"I am running for president to return our country to the principles of liberty and limited government," he says in a statement posted on the site Tuesday.
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.
Embedded in the video: an opportunity for supporters to donate.
It's unclear how much support Paul can muster in the Republican mainstream.
Paul, the son of former Rep. Ron Paul, is a frequent contrarian against his party's orthodoxy, questioning the size of the U.S. military and proposing relaxation of some drug laws that imprison offenders at a high cost to taxpayers. He also challenges the GOP's support for surveillance programs, drone policies and sanctions on Iran and Cuba.
"The issue on Sen. Paul and national security issues is where he comes down in the continuing conflict between his principles and his ambition," said John Bolton, a former ambassador to the United Nations and a potential Paul rival for the GOP nomination.
In an interview, Bolton cited Paul's shifting views on military spending.
In Paul's proposal for the 2012 budget, he called for reducing military spending and for fewer troops at the Department of Defense. "The DOD should not be treated sacrosanct with regard to the treatment of taxpayer dollars," Paul wrote in a plan that would balance the federal budget in five years.
But as a presidential campaign came closer, Paul last month proposed a 16 percent increase in the Pentagon's budget.
"On any given day, it's hard to know where he will be," Bolton said. "I believe in redemption, and I hope he comes all the way over. But I just don't know what's at work in his mind."
Perhaps reflecting the challenges he faces in convincing his critics he deserves the nomination, Paul is also leaving open the door to a second term in the Senate. With the backing of his state's senior senator, Majority Leader Mitch McConnell, Paul is likely to seek the White House and the Senate seat at the same time.
One of his likely presidential rivals, Sen. Marco Rubio of Florida, has said he would not double-dip on the ballot. He is expected to announce next week that he will skip a Senate re-election bid in 2016 in favor of putting everything into a presidential campaign.