"I've never seen so much hand movement", Trump said.
Announcing his candidacy in an online video, O'Rourke, who gained a national following with his long-shot election battle against U.S. Senator Ted Cruz a year ago, pledged to tackle "the interconnected crises in our economy, our democracy and our climate".
The first of the state-by-state nominating contests will be in February 2019.
These online observations are something to mull over while considering the many potential Democratic presidential nominees, though perhaps O'Rourke hasn't thought about it from that perspective.
His social media presence and vast fundraising power lifted him from a little-known three-term congressman to a Democratic rockstar with a following that stretched well beyond Texas.
When the Des Moines Register/CNN/Mediacom poll was first run in December, O'Rourke placed third, the top choice of 11 percent.
O'Rourke, whose given name is Robert Francis, as a child acquired his nickname "Beto", a common shortening of "Roberto" for Spanish speakers. "It's a big part of why I'm running".
Greeted by cheers when he arrived early Thursday at an event in Keokuk, Iowa, O'Rourke told the crowd it was his first visit ever to their state.
With his presidential effort, O'Rourke is hoping to leverage the fame he gained with his Senate race.
His statement continued: "Beto O'Rourke failed to get anything done in Congress, and with extreme policies like government-run health care and tearing down border barriers, his 2020 bid won't be successful either". "Yet Mr. O'Rourke also comes to the 2020 race with few notable legislative accomplishments after three terms in the House representing El Paso...."
O'Rourke is the 15th Democrat in the field and the second from Texas, and his opponents took notice.
These include 2016 election hopeful Bernie Sanders as well as Californian senator Kamala Harris.
Fellow Texan Julian Castro distributed a list of Texas Democrats who had already gotten behind the former federal housing chief's campaign. O'Rourke accused Trump of stoking "false fear" about immigrants. "I said, 'Is he insane or is that just the way he acts?'" He encouraged people to "study it".