On March 31, 2017, O'Rourke formally announced his candidacy for the United States Senate seat held by incumbent Republican Ted Cruz. In March 2018, O'Rourke became the Democratic Party nominee, winning 61.8 percent of the primary vote. O'Rourke campaigned in all of Texas's 254 counties, sometimes drawing large crowds and sometimes speaking to as few as 15 or 20 people.