O'Rourke, the latest in a growing field of candidates seeking the Democratic Party nomination, was heading to Iowa on Thursday to drum up support for his bid.
O'Rourke gained a national following with his long-shot election battle against US Senator Ted Cruz last year.
The 46-year-old former three-term US representative was a heavy underdog when he challenged Cruz, a Republican, in mostly conservative Texas, but he quickly demonstrated an ability to draw capacity crowds and raise money from voters nationwide.
His Senate bid generated a torrent of media attention and excited voters in a party desperate for fresh political faces. He lost the race by less than 3 percentage points, the tightest US Senate contest in the state in four decades.
Early opinion polls on the 2020 race have consistently ranked O'Rourke in the top tier of contenders, behind former vice president Joe Biden, who has not yet said whether he is running, and US Senator Bernie Sanders of Vermont, a 2016 presidential contender who announced his 2020 candidacy last month.
Since his Senate bid ended, O'Rourke has worked to keep himself in the public eye, regularly staying in touch with his supporters and sitting for an interview with Oprah Winfrey.