The fight for the Republican nod in Arizona’s 5th district has become just that: a fight. And most of the fighting is being done by the two leading candidates in the six-deep primary field: former Maricopa County Treasurer David Schweikert and state Rep. Susan Bitter Smith.

With most of the GOP world turning its attention today to the start of the Republican National Convention in St. Paul, Minn., many Republicans in the Phoenix-area 5th district are focused on their home turf and who will win Tuesday’s primary and the right to face freshman Rep. Harry Mitchell (D) in November.

The primary has become a muddy affair, particularly between Schweikert and Bitter Smith. One Republican with Arizona ties speculated late last week that the negative back and forth between the two frontrunners was so heated that voters might reject both of them and choose one of the other four candidates, possibly lobbyist Jim Ogsbury.

“Bitter Smith and Schweikert appear to be aware that they are the two frontrunners,” said the GOP insider. “But if they continue down their current path, one could foresee a situation where the door is open for Ogsbury.”

Also running in the GOP primary are state Rep. Mark Anderson, former state Rep. Laura Knaperek and business consultant Lee Gentry. Republicans have a 40,614 voter- enrollment advantage in the 5th district, explaining why so many Republicans are jockeying to challenge Mitchell, who defeated then-Rep. J.D. Hayworth (R) in 2006.

But the late primary date benefits Mitchell, who closed the pre-primary fundraising period on Aug. 21 with a healthy $1.3 million in cash on hand. Although the Congressman is just a freshman, he has served in the state Legislature and as Tempe mayor, and he has been a popular elected official in various parts of the district for several years.

As of Aug. 21, Schweikert had spent $699,000 on his campaign, which included a personal loan of $250,000. He closed the pre-primary fundraising period with $244,000 on hand. Bitter Smith raised nearly $181,000 for the cycle, including a personal loan of $150,000. She reported $82,000 in the bank.

Schweikert last ran for Congress in 1994, when he lost in the 5th district GOP primary to Hayworth. Schweikert campaign manager Bill Connor confirmed that his internal polling has shown this contest to be one mainly between his candidate and Bitter Smith, with all other candidates garnering support in the single digits.

Connor said he feels good about Schweikert’s prospects. And he acknowledged that the competition with Bitter Smith has gotten somewhat testy.

“We’ve put enough resources behind the message,” he said. “It’s a fight that is near and dear to our hearts: whether we’re going to put a real conservative up against Harry Mitchell or not.”

Bitter Smith is similarly optimistic about her chances of prevailing Tuesday.

She agreed that the campaign had taken a nasty turn in recent days. But she blamed the development on the Club For Growth — which is backing Schweikert and advertising on his behalf — and Schweikert himself.

She said that the club and the Schweikert campaign have been hurling falsehoods and that she had no choice but to react.

“At some point you have to respond back with facts,” Bitter Smith said in an interview Thursday. “We’ve had to talk about his record as county treasurer.”

Like Schweikert’s polling, Bitter Smith said her data also shows a tight race between herself and Schweikert, with the rest of the field out of contention.

Bitter Smith’s media campaign has included a heavy dose of direct mail, in addition to a presence on talk radio and cable and broadcast television advertising. Schweikert has relied more on broadcast and cable television, but he is also advertising through the mail and on the radio.

Publicly, Republicans are talking as though Mitchell is a top target in the fall elections. But privately, they have acknowledged that just how much effort they put into this race could depend on the perceived strength of their nominee.

Mitchell certainly believes that the Republicans are coming after him. However, the Congressman said he is prepared.

“I think it’s a real tough race. They are targeting [me], from everything I understand,” Mitchell said last week in Denver, while attending the Democratic National Convention. “But saying all of that, I think we’re in a very good position. We’re ready.”

Camilla Strongin, a spokeswoman for the Arizona Republican Party, described the 5th district general election campaign as “one to watch.” She noted that the GOP has an edge among registered voter and predicted that the eventual Republican nominee would give Mitchell trouble this fall.

“Whoever wins the Republican primary will give Harry Mitchell a serious run for his money,” Strongin said.