Dewhurst, 66, who presides over the state senate, has put $19m of his own money into the race, his campaign said. He also has the backing of top Texas Republicans including Governor Rick Perry. Dewhurst is a former state land commissioner who has served in the U.S. Air Force and with the Central Intelligence Agency.

He has dismissed Cruz as not having enough experience and running a campaign that depends on "Washington insiders."

In Cruz's corner are national conservative stars such as former Alaska Governor Sarah Palin and national groups such as the Club for Growth and FreedomWorks that have funneled money and volunteers to his campaign.