By Corrie MacLaggan
AUSTIN, Texas (Reuters) - Tea Party favorite Ted Cruz scored a stunning upset over a longtime Texas state officeholder in a Republican U.S. Senate primary runoff on Tuesday, transforming Cruz into a national conservative star and marking a resurgence of the movement to shrink the size of U.S. government.
Cruz, 41, a former state solicitor general who has never held elected office, became the third insurgent Republican this year to defeat an establishment Republican in a U.S. Senate primary.
He scored a surprisingly comfortable victory with about 56 percent of the vote to about 44 percent for Lieutenant Governor David Dewhurst, who a year ago was considered the frontrunner.
"They said this was impossible," Cruz told a roaring crowd of supporters in Houston on Tuesday night. "They said I couldn't do it. And you know, they were right. I couldn't do it, but you could and you did it. Tonight is a victory for the grass roots."
Cruz will be a strong favorite to win the election in November against Paul Sadler, who won the Democratic primary on Tuesday, because Texas has not elected a Democrat to statewide office since 1994. The Senate seat is being vacated by Republican Kay Bailey Hutchison.
Most Texans had never heard of Cruz when he took on Dewhurst, 66, a wealthy businessman who spent $19 million of his own money on the race and had the support of top Texas Republicans including Governor Rick Perry.
"When Ted gets to Washington, he's going to be seen correctly as a giant-killer," said Sal Russo, co-founder and chief strategist of Tea Party Express, the nation's largest Tea Party political action committee.
Cruz spent a year and a half crisscrossing the state, introducing himself at Tea Party meetings and Republican women's club gatherings as a "constitutional conservative."
And he drew support from conservative stars such as former Alaska Governor Sarah Palin and money from national conservative groups such as the Club for Growth. That group's political action committee spent $5.5 million to support Cruz, the organization said.
Cruz, whose father came to Texas from Cuba with $100 sewn into his underwear, would become the first Hispanic U.S. senator from Texas if he wins in November.
Dewhurst and Cruz had similar policy positions. Both pledged to do away with President Barack Obama's health care reform and to rein in Washington spending.
Cruz, a Princeton University debate champion and a Harvard Law School graduate, stood out with sharp debate performances. Some Cruz voters said they viewed Dewhurst, a former state land commissioner who has presided over the state senate since 2003, as something of an incumbent who had been in office too long.
Dewhurst received the most votes in a crowded field in the first round of the primary, but he did not secure the 50 percent plus one vote he needed to avoid a runoff with Cruz.
"We came up short, and I am not used to coming up short," Dewhurst, who stands 6 feet 6 inches tall, told supporters Tuesday night in Houston.
Cruz's primary victory follows that of Richard Mourdock, the Tea Party movement-backed candidate who defeated longtime U.S. Senator Richard Lugar in the Republican primary in Indiana.
Since Mourdock's primary win, and another Republican primary upset victory in Nebraska with rancher Deb Fischer, who beat veteran attorney general Jon Bruning in a U.S. Senate primary, insurgent Republican groups have hoped for a similar story in Texas.
Some national political analysts had predicted the Tea Party movement, which sprang out of the 2008-2009 recession and advocates a small federal government, would be a diminished force in the 2012 election.
"The big thing that I think this demonstrates is that the Tea Party is far from gone -- it's truly alive and well," Russo said of Cruz's win.
(Additional reporting by Patricia Gras and Deborah Quinn Hensel in Houston; Editing by Greg McCune, Stacey Joyce and Lisa Shumaker)