Topeka ? Hoping to cash in with so-called social conservative Republicans, U.S. Sen. Sam Brownback of Kansas is turning up the heat on fellow Republican Mitt Romney in the GOP presidential contest.

Much of Brownback’s campaign in recent days has been aimed at criticizing Romney on the issue of abortion as the candidates prepare for Saturday’s Iowa Straw Poll.

On Tuesday, Brownback’s national campaign manager, Rob Wasinger, said Romney “has flip-flopped so much on the issue of abortion that he has absolutely no idea where he stands on the issue.”

In a testy exchange during a debate Sunday, Romney denounced Brownback’s automated phone call campaign, which notes Romney’s earlier pro-choice position.

“I get tired of people that are holier than thou because they’ve been pro-life longer than I have,” Romney said.

The jousting occurs as many of the lesser-known Republican candidates, such as Brownback, hope to get a boost from the straw poll in Ames.

In the straw poll, candidates buy tickets for their supporters to cast votes for them. The ticket sales are used as a fundraiser for the Iowa Republican Party. The event is often seen as a test of candidates’ strength five months before the Iowa caucus.

GOP candidates Sen. John McCain of Arizona and former New York Mayor Rudy Giuliani aren’t participating in the straw poll, although their names will appear on the ballot.

Hoping to increase his national recognition, Romney is campaigning hard in the straw poll. But that has lower-tier candidates gunning for him.

Brownback also announced Tuesday that Norma McCorvey, an anti-abortion activist, will support him. McCorvey was the ”Jane Roe” in the 1973 Roe v. Wade lawsuit, in which the U.S. Supreme Court legalized abortion. She has since abandoned her pro-choice position and fought to end abortion.

“Sam Brownback is the right person in the right place at the right time for our nation as we seek to embrace a culture of life,” McCorvey said.