Harris fundraised off O'Rourke's entrance into the race, sending an email to supporters that says she looks "forward to engaging in substantive debates with each of these candidates - including the newest to join the race today, Beto O'Rourke - and ultimately selecting a Democratic nominee who will take on and beat Donald Trump in November of 2020". "This city is the best example of this country at its best".
Beyond a previously announced event in Waterloo, Iowa, on Saturday, O'Rourke is expected to visit other towns in eastern Iowa, traditionally a Democratic stronghold.
Trump took questions from the Oval Office during his meeting with Irish prime minister Leo Varadkar, and he was asked for his thoughts on O'Rourke's campaign announcement.
O'Rourke, 47, served three terms in Congress representing a region of West Texas along the USA border with Mexico.
Mr O'Rourke was little-known outside his hometown of El Paso until he challenged Republican Senator Ted Cruz previous year.
"I think that John Cornyn is happy no one of Beto's status is going to run against him".
The addition of O'Rourke makes a crowded Democratic field even more crowded, and it will be fascinating to see if he can find a constituency, and where it will come from.
In his six years in Congress representing part of West Texas along the border with Mexico, O'Rourke largely did not ally himself with progressives, instead joining the centrist "New Democrat" coalition.
Vanity Fair quoted O'Rourke as saying in an interview published on Wednesday that he was aware of his disadvantage as a white man at a time when many in the Democratic Party want a woman or a person of color for president. "I think there's going to be people that. have their own money".
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 announced whether he is running, and U.S. Senator Bernie Sanders of Vermont.
"Whereas Obama is from the tail end of the baby boom, Beto O'Rourke is quintessentially Generation X, weaned on Star Wars and punk rock and priding himself on authenticity over showmanship and a healthy skepticism of the mainstream".