Welcomed by roaring applause, Sen. Rand Paul (R-Ky.) launched his presidential campaign in Louisville, Ky., by vowing to "take our country back." (AP)

This story has been updated.

LOUISVILLE -- Sen. Rand Paul, the maverick first-term senator who rode a tea party wave from a Kentucky ophthalmology practice to Congress, on Tuesday formally announced a bid for the 2016 Republican presidential nomination.

"I have a message, a message that is loud and clear and does not mince words. We have come to take our country back!" Paul told a crowd at his official campaign launch in Louisville, echoing language he used during his 2010 Senate run. "We have come to take our country back from the special interests that use Washington as their personal piggy bank, the special interests that are more concerned with their personal welfare than the general welfare."

Standing on a stage flanked by American flags, Paul emphasized a message of inclusion and diversity in step with his strategic aim to expand the traditional GOP base.

"The message of liberty, opportunity and justice is for all Americans, whether you wear a suit, a uniform or overalls, whether you're white or black, rich or poor," Paul said. "Many Americans, though, are being left behind. The reward of work seems beyond their grasp. Under the watch of both parties -- the poor seem to get poorer and the rich get richer."

Paul’s announcement makes him the second major Republican candidate to officially jump into the 2016 race, just the first of a string of campaign debuts slated for this month.

A fiercely independent conservative, Paul has drawn notice from the start of his tenure on Capitol Hill, clashing with GOP leadership and introducing his own proposed overhaul for the federal budget. In 2013, the nearly-13-hour filibuster he conducted in opposition to President Obama’s drone policy and the nomination of John O. Brennan to head the CIA catapulted him to stardom on the right, especially among younger Republican voters who have craved a new, contrarian voice.

Paul frequently knocked the Republican Party throughout his remarks Tuesday, accusing his own party of being part of the problem while calling for new, smaller-government policies to spur economic growth.

"Too often when Republicans have won, we've squandered our victory by becoming part of the Washington machine. That's not who I am. That's not why I ran for office the first time just a few years ago," he said. "... It seems to me that both parties and the entire political system are to blame. Big government and debt doubled under a Republican administration. And it's now tripling under Barack Obama's watch."

An ophthalmologist who still performs pro bono eye surgeries, Paul did not hold elected office until ascending to the Senate in 2010 -- but he’s been steeped in politics for his entire life as the scion of one of America’s most famous libertarian families. As a college student, Paul once took a semester off from school to work on father Ron Paul’s Senate primary campaign in 1984 and was involved with the longtime Texas congressman's other runs for elected office, including his two campaigns for the GOP presidential nomination.

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On the campaign trail, the Kentucky senator is expected to continue the pitch he’s made while traveling the country to lay the groundwork for a presidential run: that the Republican Party needs to broaden its appeal to voters who have traditionally shunned it.

“White, black, brown, rich, poor, with tattoos and without tattoos, with earrings and without earrings,” Paul told a room packed full of young people in February at the Conservative Political Action Conference in Oxon Hill, Md. “We need to take our message where it’s not been before.”

Days before he was expected to announce his candidacy for president, Sen. Rand Paul (R-Ky.), released a video with footage from news clips covering him and from his speech at the 2015 Conservative Political Action Conference. (Rand Paul/YouTube)

On Capitol Hill, Paul has been known for his unlikely alliances with Democrats. He has co-authored bills on reforming the criminal justice system with Sen. Cory Booker (D-N.J.) and legalizing medical marijuana with Booker and Sen. Kirsten Gillibrand (D-N.Y.). Criminal justice reform has become one of Paul’s signature issues, and he has been using it, and a plan to boost economic investment in Detroit, to try to make inroads with black voters.

“There is still significant segregation in our society,” Paul said last month at Bowie State University, a traditionally black university in Maryland.

And Paul has repeatedly sparred with some of his GOP colleagues, including both Sen. John McCain (Ariz.) and likely challenger Sen. Marco Rubio (Fla.) over foreign policy and other issues. But as he has approached his presidential run, Paul has been mending fences with many Republican colleagues, including his state's senior senator, Majority Leader Mitch McConnell. McConnell has endorsed Paul’s push for a Kentucky presidential caucus, which would allow the junior senator to run for president and his Senate seat simultaneously.

Paul has been looking to broaden his appeal outside of Washington as well, with some of his more notable libertarian stances inching into more traditionally conservative territory as he prepared to run for president. He has been meeting with evangelical pastors to woo the faith community in the important states of Iowa and South Carolina; last month, he told a group of pastors that a moral crisis is leading people to believe that same-sex marriage is acceptable.

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On foreign policy, his tone has softened and his once strident non-interventionist worldview has been shifted away from its perch of ideological purity, with Paul now frequently acknowledging the necessity for a strong national military and the threat of terrorism in the Middle East, in particular in the Islamic State. Still, he continues to court conservatives who became war weary during George W. Bush’s presidency, knowing they are critical to his national support.

Paul has also been tirelessly reaching out to young people, speaking at colleges and universities around the country for the last year and a half. According to a Washington Post/ABC News poll, Paul runs three times as well among voters under 50 than over 50. And he's staking a claim to tech during the 2016 cycle, opening offices in Austin and Silicon Valley and using platforms like Snapchat to win over younger voters who may not respond to traditional political ad buys or outreach efforts. Paul is also trying to tap into the vast well of tech money, holding meetings with big-pocketed potential donors in California and other tech hubs.

Sen. Rand Paul, (R-Ky.), who announced he's running for president in 2016, is known for his belief in limited government. Here his take on Obamacare, the Constitution and more, in his own words. (Julie Percha/The Washington Post)

Paul's formal campaign launch was at noon at the Galt House, a hotel in downtown Louisville. Over the rest of the week, he plans to barnstorm the four early voting states -- New Hampshire, South Carolina, Iowa and Nevada -- in a "Stand with Rand" tour. An adviser said Paul will hit on different themes at each stop: In New Hampshire on Wednesday, he will discuss civil liberties, while on Thursday in South Carolina he will focus on national defense in a speech at the USS Yorktown.

As his campaign kicks off, Paul will be surrounded by many of the advisers who helped him ascend to the Senate, including his closest aide, Doug Stafford, and longtime Paul family loyalist Jesse Benton. He'll also be joined by Ron Paul, who is not not scheduled to speak.

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The senator lives in Bowling Green, Ky., with his wife, Kelley. They have three sons. He attended Baylor University, but did not graduate, and Duke Medical School. At the time, Duke didn’t require medical students to have an undergraduate degree, a policy that has since changed.

Paul will publish two books this year: “Taking A Stand” will come out May 26, and “Our Presidents and Their Prayers” will be released in the fall.

"It's time for a new way, a new set of ideas, a new leader," Paul said at CPAC in February, in footage he used in a video released Sunday night. "Above all, it's time for a new president."

Jose A. DelReal contributed to this report. He and Costa reported from Washington.

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