Eighteen months ago, Ted Cruz was a starry-eyed Texas Republican with long-shot hopes of becoming a United States senator. On Tuesday, the former state solicitor general looks headed to an unlikely runoff victory over Lt. Governor David Dewhurst, win that would defy the power of the state’s GOP establishment.

After speaking in support of Ted Cruz, Sarah Palin signed autographs and spoke to some of the 1,000 Ted Cruz supporters during a rally for Cruz, Friday, July 27, 2012 in The Woodlands, Texas. (AP Photo/Houston Chronicle, Johnny Hanson)

There are three key reasons for this.

For starters, Dewhurst didn’t implode. He raised heaps of money and added millions from his own checkbook. He began with a huge name identification advantage, was backed by Gov. Rick Perry, and enlisted the help of David Carney, one of the sharpest political minds in Texas. And he was the beneficiary of a super PAC headed by Rob Johnson, another top Texas GOP hand.

By contrast, Sen. Richard Lugar (R-Ind.) ran a lackluster operation which was slow to respond to a lingering story about his out-of-state residency and plagued by a record that enraged conservatives. In Nebraska, frontrunning GOP Attorney General Jon Bruning was beleaguered by ethics questions and gaffe-prone behavior. The errors both men made opened them up to upstart challengers. Dewhurst didn’t make those same sort of mistakes.

Both Bruning and Lugar were also dogged by charges of being insufficiently conservative. Dewhurst was too, but those assaults were less fair. Bruning supported confirming Eric Holder as Attorney General while Lugar’s work across the aisle won him (toxic) praise from Democrats. But Dewhurst showed no signs he’d be anything but a reliable Republican vote in the Senate.

So how did Cruz stay competitive and even climb into what looks like the driver’s seat? He leveraged national acclaim from conservatives like Sen. Jim DeMint (R-S.C.) and George Will into strong local tea party support, winning an impressive 34 percent of the primary vote and forcing a runoff against Dewhurst, who took 45 percent in the late May election.

And he’s proving to be a great closer. He outraised Dewhurst more than 3-1 during the first third of the month, spurred Sarah Palin and DeMint to stump in the state for him last Friday and never lost the confidence of the anti-tax Club For Growth, which has spent $5.5 million on independent expenditures to help him win.

But don’t write Dewhurst off just yet in a race strategists on both sides expect to be close. An internal poll showed him holding a slight lead last week. A review of ad spending during the final week reveals that Dewhurst and his allies outspent Cruz and his supporters by nearly $1.3 million on TV. And early voting – which was good to Dewhurst in the primary – was robust in the runoff.

“I think the turnout is going to be higher than what most people are projecting,” said Dewhurst spokesman Matt Hirsch.

Still, it’s difficult to overlook the signs Dewhurst is in trouble. The super PAC supporting him released a harsh TV ad last week smacking of desperation that tried to tie Cruz to the suicide of a young man. Meanwhile, like Dewhurst, Cruz has also released polling showing him ahead; both sides agree that Dewhurst’s double-digit lead is long gone.

“The momentum and enthusiasm really seems to be with Ted Cruz,” said Rice University political scientist Mark Jones.

A Cruz victory would suggest that Republican voters aren’t just rejecting inadequately conservative candidates and longtime incumbents. The sufficiently conservative who are tied the establishment — a word becoming more and more toxic in politics — are also at risk.

A Tuesday win by Cruz would mean just about every candidate left in a GOP primary who is challenging a frontrunner tied in ANY way to the establishment will do everything they can to compare their candidacy to Cruz’s.