"Today, I announce with God's help, with the help of liberty lovers everywhere, that I am putting myself forward as a candidate for the United States of America," Paul said at an event in Louisville, Kentucky.
Over the past year, Paul, 52, has tried to expand the reach of the Republican Party by appealing to a diverse demographic, going into urban communities to court African American voters, courting Silicon Valley donors, and speaking to young college students on privacy issues.
"This message of liberty is for all Americans, Americans from all walks of life," he said. "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, who is the second major candidate to formally enter the 2016 race, will soon embark on a fly-around tour of the country's early primary states -- hopping from New Hampshire and South Carolina to Iowa and Nevada by week's end to sell his brand of Republican politics.
Here are five obstacles standing between Paul and the White House:
1. His Father's ShadowThis isn't the first time a member of the Paul family has run for president. His father, former Rep. Ron Paul, R-Texas, sought the presidency three times (1988, 2008 and 2012). The elder Paul's vocal anti-war, libertarian leanings became a sort of movement, building a network of supporters the younger Paul can draw on in his own presidential campaign. But while his father injected his politics into the Republican presidential primaries, Rand Paul hopes to make it past the primaries into the general election. To do that, he will have to distance himself from some of his father's views in order to show he's not just an extension of Ron Paul and win over a broader slice of the Republican Party.
2. How Libertarian Will Paul Be?The young Kentucky ophthalmologist was elected to the Senate during a tea-party wave, and he quickly injected a dose of libertarian-leaning politics into the Senate, speaking out on issues such as government surveillance and foreign policy. Most famously, in 2013, he launched a 13-hour filibuster against the nomination of John Brennan to be director of the CIA as a means of opposing the administration's use of drone strikes. But as he embarks on a 2016 campaign, Paul, who has described himself as "libertarian-ish," will have to strike a delicate balance between appealing to libertarian supporters, like those who backed his father, and not alienating establishment Republicans and social conservatives.
3. The Foreign Policy QuestionOne big question will be how Paul plays in the foreign policy sphere. When he joined the Senate, he frequently found himself at opposite ends of the Republican base on issues relating to foreign policy. For instance, he often engaged in foreign policy fights with Sen. John McCain, the most hawkish Republican in the Senate. But over time, Paul's tone on foreign policy and defense has shifted -- leaving many to wonder where exactly he'll fall in 2016. Case in point: military spending. When he joined the Senate in 2011, he unveiled a budget to slash defense spending by nearly $10 billion in 2016. But last month, he introduced a measure that actually increased defense spending by $190 billion over the next two years.
4. Running for 2 Positions at OncePaul is already tackling one of his biggest obstacles to the presidency head-on -- a Kentucky law that prohibits candidates from appearing on the same ballot twice. In addition to running for the presidency, Paul plans on running for a second Senate term in 2016. The Kentucky senator has asked the state's Republican Party to change its nominating contest from a primary to a caucus -- a move that would allow Paul to run for both offices because caucuses don't require ballots. His likely opponent, Sen. Marco Rubio, R-Florida, faces the same conundrum but has said he will only run for one office next year.
5. He Comes From the SenatePaul is trying to move from a Senate career to the top job at the White House -- something only 16 other presidents before him have done. Five of the last seven presidents were governors, meaning it's a tough climb for senators with their eyes on the White House in 2016. But hey, if a little known senator from Illinois named Barack Obama was able to win the presidency after just four years in the Senate, maybe Paul can, too.