The Arizona Republican Senate primary features a former Republican congressman challenging the party's most recent presidential nominee, and it's so far proven to be one of the more interesting races of the year -- this week especially.

The campaigns for Sen. John McCain and J.D. Hayworth have consistently traded shots -- that McCain isn't conservative enough and that Hayworth isn't the fiscal conservative he says he is. Both have congressional voting records the other can attack, and both have jumped at that opportunity.

The Hayworth campaign says McCain is "vulnerable to the more conservative Hayworth on taxes, the Second Amendment, pro-life and pro-family issues and illegal immigration." Hayworth has run a radio ad highlighting his embracement of Christianity, "unlike some leaders who shy away from their faith." Another radio ad calls Hayworth the only "consistent conservative" in the race.

Hayworth gained plenty of press this week for stating in a radio interview that the Massachusetts Supreme Court's decision in 2004 to allow gays and lesbians to marry was vague enough to imply that a human could marry a horse. It played into the McCain campaign's depiction of Hayworth as not fit for the Senate.

The campaign had already tied Hayworth to the birther movement, which believes President Obama is not a U.S. citizen and therefore cannot legally be president.

Meanwhile, McCain is bringing out the big guns: He brought in Mitt Romney this week, and next week Sarah Palin will appear with him in Tucson and Phoenix.

The latest battle du jour has erupted over earmarks. Hayworth recently slammed McCain for voting for the 2008 bank bailout, which "included $150 billion in pork." McCain retaliated with a web video in which the announcer says: "When it came to pork barrel earmarks, J.D. Hayworth just couldn't stop voting yes." McCain's video calls out Hayworth's support for the 2005 highway bill that included such projects as the infamous "Bridge to Nowhere."

The Hayworth campaign called the ad "misleading" and a distorting of his record. The McCain campaign then responded to that, calling Hayworth a "big spending Republican."

McCain also boasts the support of Rep. Jeff Flake (R-Ariz.), who is McCain's maverick counterpart in the House. Flake, Hayworth's former colleague in the House, has long opposed the earmarking process, and this week criticized Hayworth's participation in it.

"While Senator McCain has offered countless bills and amendments in the U.S. Senate to address this problem, Congressman Hayworth's position on earmarks was part of the problem," Flake said.

A Rasmussen survey released this morning found the race close, with McCain getting 48 percent support to Hayworth's 41 percent. That's a low number for an entrenched incumbent like McCain.