The 46-year-old O'Rourke, a former three-term USA congressman and a punk rocker in his youth, pledged to tackle "the interconnected crises in our economy, our democracy and our climate".
'You can probably tell that I want to run, ' he says.
Several hours after announcing his bid for the White House, former Rep. Beto O'Rourke, D-Texas, defined one significant aspect of his platform, saying he'd suspend capital punishment at the federal level.
O'Rourke said he will hold a kick-off rally for his campaign on March 30 in El Paso, Texas.
He took a well-publicized road trip across the American Southwest, stopping at colleges and diners.
O'Rourke has come under fire from some liberals in the months since his Senate race ended for appearing to break a pledge not to accept money from fossil fuel companies.
Asked about "Beto" getting into the race by Fox News, White House Principal Deputy Secretary Hogan Gidley was dismissive of O'Rourke's campaign logo- and merchandise-friendly nickname.
O'Rourke hit the campaign trail on Thursday in Iowa.
The former congressman had been testing the waters for a presidential run in unconventional ways.
"I had to be moving, I had to be meeting people", O'Rourke told Oprah Winfrey in a February interview. I think she is a very effective campaigner.
Weeks of teasing an announcement are behind him, but O'Rourke now must prove whether the energy he brought to the Texas campaign will resonate on a much larger stage.
While running for Senate, he offered an unapologetically liberal vision, supporting Trump's impeachment, universal health care, gun control, marijuana decriminalization, steep federal subsidies for prekindergarten education and relaxed immigration policies. And while O'Rourke took a series of progressive positions - he argued for criminal justice revisions and marijuana legalization, backed "Medicare-for-all" and said he would support President Donald Trump's impeachment - the race was primarily a clash of personalities.