Republicans are bracing for what could be a nasty primary in the race to replace retiring Sen. Jon Kyl (R).

Rep. Jeff Flake remains the frontrunner — both in the GOP primary and in a general election that Democrats are increasingly optimistic they can make competitive.

But before Flake can begin to focus on Democrat Richard Carmona, he faces what looks to be an interesting primary against wealthy real estate investor Wil Cardon.

Thanks in large part to his deep pockets and his self-funding thus far, Cardon has steadily picked up credibility. The latest numbers show he has put $4.2 million of his personal fortune into his campaign. So far, about $1 million of that has gone toward television buys.

“You can see that now that folks know there’s another alternative to Congressman Jeff Flake, you can see the support is switching,” Cardon spokeswoman Katie Martin said.

Flake has yet to go on television. He will between now and the Aug. 28 primary, but his campaign declined to say what its timeline is.

“I just don’t see it at all,” a source in the Flake campaign countered. “The guy has spent a million dollars. We do a lot of polling work, and it’s not having any kind of real effect of moving the ball forward.”

One odd angle to the dynamic is how little attention the Senate race has actually garnered.  Typically, Senate campaigns dwarf House races in interest, but not so in Arizona.

Redistricting upset the state’s political stability. The state features three tossup House races and several high-profile primaries, including a Member-vs.-Member race between GOP Reps. Ben Quayle and David Schweikert. There is also the special election to replace former Rep. Gabrielle Giffords (D) and the continuing drama surrounding 4th district GOP candidate Paul Babeu’s personal life.

Add to that the fact that President Barack Obama’s campaign has begun to make noise about putting Arizona in play, and one can see why oxygen for the Senate race is sparse.

TV air time will be even harder to come by in the summer. The Phoenix market specifically will be inundated with political ads from competitive House primaries and other candidates looking to the fall.

But Cardon has been on air early and often this spring.  

Flake’s team has begun to respond to the Cardon camp in recent weeks with a series of press releases called “Fact Checks” criticizing Cardon statements. Those outside of Flake’s campaign point to that as a sign that the Congressman is taking him seriously.

In the context of a normal campaign, Flake has had strong fundraising. This past quarter he raised $935,000 and ended March with more than $3 million in the bank. To help combat Cardon’s self-financing, Flake has an effective fundraising ally in the Club for Growth.

“We fully expect him to be the next Senator from Arizona,“ Club spokesman Barney Keller said.

Cardon has obviously sought to offset the Club’s involvement with what he calls the “Arizona First Pledge.” On his website, he displays a letter to Flake seeking a ban on spending from outside groups, similar to the agreement in the Massachusetts Senate race between Sen. Scott Brown (R) and Harvard professor Elizabeth Warren.

Flake has yet to respond to the letter.

Democrats, for their part, delight in the notion of Cardon creating trouble for Flake.

“Jeff Flake is a career politician who has never had a competitive election, and between his flip-flops on immigration reform to his work as a lobbyist, he has a lot of vulnerabilities that have never been scrutinized in the context of a real political campaign,” Democratic Senatorial Campaign Committee spokesman Matt Canter said.

Rep. Ed Pastor said he has seen Cardon’s ads and argued that at the very least, from a financial standpoint, Republicans can’t be happy about the primary.

“I was in Tucson two weeks ago, and he was playing in Tucson. So he’s identifying himself,” the Arizona Democrat said. “I would think with [Cardon] being a self-funder — that Jeff is going to have a primary, and by having a primary, it means your monies are diminished.”

But those in Flake’s camp maintain that Cardon will do little damage to the Congressman’s general election prospects.

Republican strategists, meanwhile, are conflicted over whether the Arizona seat is actually in play. Most agree, though, that unless the primary grows increasingly hostile, it will have little effect on the general election.

Cardon’s main line of attack is on immigration, an issue on which many hard-line conservatives perceive Flake as soft. His website labels the Congressman as “Mr. Amnesty.” One national strategist said that the issue could be “an albatross” for the lawmaker.

The Flake source noted that polling data shows that immigration, while still an important issue, is not on the voters’ minds like it was in 2010. Pocketbook issues like health care and the deficit are more prominent.

Flake supporters maintain that he cannot be outflanked on fiscal issues, and part of their strategy seems to be in painting Cardon as too extreme.

Cardon recently hedged when asked if Obama is a natural born citizen, according to the Arizona Republic. But the Cardon campaign responded vigorously to Roll Call when pressed on the matter.  

“Wil Cardon believes that he [President Obama] was duly elected to the office of the President of the United States,” Martin, Cardon’s spokeswoman, said.

In 2010, Sen. John McCain (R) used comments by former Rep. J.D. Hayworth on the birther issue to portray him as too extreme in their GOP primary. McCain ultimately won handily.

Cardon has four months to make his case to voters. Most Republicans assume he will not be able to close the gap with Flake. But if Cardon continues to spend freely, he would not be the first self-funder to create trouble for a frontrunner.

Daniel Newhauser contributed to this report.