Is Gabrielle Giffords’s (D-Ariz.) former aide the odds-on favorite to claim her seat in the upcoming special election? Probably.

Is it a slam dunk? Hardly.

Voting begins in the special election today, with Republicans picking a nominee to face former Giffords aide Ron Barber (D) in the June 12 special election. And for a while now, the assumption has been that Barber would probably win the race — thanks in no small part to his close ties to Giffords. (For more on Barber, see former Fixer Felicia Sonmez’s profile from last week.)

Ron Barber, left, a former top aide to former congresswoman Gabrielle Giffords, right, is favored to win a special election for her seat. But he’s running in a Republican-leaning district. (AP)

But if the past several years have shown us anything, it’s that special elections are rarely predictable, often surprising and usually a big deal.

Arizona’s 8th district could be the latest to fit that mold.

While there is undoubtedly plenty of goodwill for Giffords, and Barber (who was wounded in the same shooting that nearly took his boss’s life) has shown himself to be a very strong fundraiser early on, this is still, quite simply, a Republican-leaning district.

The district voted 52 percent for Sen. John McCain (R-Ariz.) in the 2008 presidential election, 53 percent for President Bush in 2004, and 38 percent of voters are registered Republicans, compared to 32 percent who are registered Democrats.

In fact, it’s one of the most Republican districts in the country that didn’t go the GOP’s way in the 2010 election, in large part because Giffords played her cards right and built a profile as a popular Blue Dog Democrat over her first two terms in office.

“I think (Barber) does have an advantage in that he’s seen as filling out her term,” said local GOP pollster Margaret Kenski. “However, the more it gets conflated with the presidential race and the Senate race ... the better it is for Republicans.”

In her last race, Giffords appeared to benefit big time from a late and contested GOP primary in which Iraq veteran Jesse Kelly emerged over the establishment favorite, former state senator Jonathan Paton. She won early voting by 12,000 votes, lost Election Day voting to Kelly by 8,000 votes, and emerged victorious by 4,000 votes overall.

Two years later, Kelly is back and has largely avoided the rough-and-tumble primary that dogged him in 2010. And, in fact, a Republican-leaning pollster on Monday released a poll showing the likely GOP nominee leading Barber 49 percent to 45 percent.

Even if you concede that the National Research poll may be more favorable to the GOP than the race is in reality, it demonstrates the terrain that the race is being fought on.

“What the poll reflects, quite frankly, is naturally where the district is – right of center,” said a Republican strategist granted anonymity to speak candidly about the race.

But the poll doesn’t paint a great picture for the GOP, either. It shows that Kelly and Barber have about the same favorable ratings (44 percent view Barber favorably, compared to 43 percent who like Kelly), but Kelly’s unfavorable rating (40 percent) is considerably higher than Barber’s (24 percent).

Much of this, undoubtedly, is left over from Kelly’s 2010 primary and his campaign against Giffords, in which he started out as a tea party insurgent candidate and seemed to pay a price for it in the general election (particularly in early voting). This time, he has run a more soft-spoken and measured campaign, thanks in part to compliant opponents.

And the fact that Kelly has worse numbers but still leads the poll says something about the makeup of the district.

“I don’t think anybody thinks (Barber) can lose, but funky things do happen,” said an Arizona Democratic strategist.

The poll suggests that the race, if nothing else, is competitive to start with. Considering that Kelly’s unfavorable rating is already pretty high, we can expect Republicans to go after Barber hard and try to drive up his negatives. The state Republican party has already spent more than $110,000 trying to define Barber through a series of mailers.

That process begins more publicly after tonight’s primary, when Kelly is favored to beat a field that includes state Sen. Frank Antenori, former Air Force pilot Martha McSally and broadcaster Dave Sitton.

“His message of lower taxes and more jobs is resonating throughout the district,” Kelly adviser Ed Brookover said. “He has been reaching out to voters from every segment in southern Arizona in his campaign for a better Arizona.”

Barber spokeswoman Jessica Schultz said her boss is ready.

“The reality is, no matter who emerges on the primary, we’re going to run a strong race on jobs and the economy, middle-class families,” Schultz said.

At the same time, the GOP’s task in defining Barber is made significantly harder by virtue of his association with Giffords. Given what happened in the district in early 2011, there may not be the same appetite for negative or harsh campaign tactics. And Barber, for his part, is emphasizing civility in his campaign.

Republicans will really have to thread the needle, then.

But they are fighting on friendly terrain, and in a low-turnout special election, basically anything can happen.