"The conservative groups that were involved in those races are energized and all of that firepower is now focused on the race in Texas," said Drew Ryun, president of American Majority Action, a conservative group that trains activists.

But the Texas race is somewhat different. Indiana's Lugar was accused of being out of touch with the state, and ethical problems dogged Nebraska's Jon Bruning; Dewhurst's candidacy does not have such serious flaws, Jones said. But critics argue that Dewhurst is not a dynamic leader and that he has compromised with Democrats to get things done in the Texas Senate, Jones said.

"That has been his record over and over again: that he's a conciliator, he cuts the baby in half, he's increased spending," Cruz said in an interview on Thursday.

Dewhurst, who as lieutenant governor has presided over the Texas Senate since 2003, said after a campaign event at the Scholz Garten bar and restaurant in Austin on Wednesday that he has been the most fiscally and socially conservative lieutenant governor in Texas history.

Dewhurst, a former state land commissioner who stands 6-feet 6-inches tall, touts his experience as a businessman as well as serving in the U.S. Air Force and with the CIA.

In Houston on Thursday evening, Cruz told enthusiastic supporters at the Armadillo Palace restaurant that the race was going "incredibly, dizzyingly, unbelievably" well. Database analyst Henry Wycislo explained why he came to the campaign event: "Tea Party all the way."

Political scientist Jones said that if Dewhurst is forced into a runoff on July 31, summer turnout will be low and the fervor among Tea Party supporters could help Cruz.

But Dewhurst criticized the support Cruz is receiving from groups outside Texas. "One thing that Mr. Cruz is sadly mistaken on: He ain't met a fighter before like me," Dewhurst said.

(Additional reporting by Nick Carey; Editing by Greg McCune and Eric Beech)