In their first debate Tuesday, Kansas Reps. Jerry Moran (R) and Todd Tiahrt (R) both assailed each other's congressional voting records.

The two are locked in a Republican primary for the nomination to fill the seat of Kansas Sen. Sam Brownback (R), who is running for governor. The primary is Aug. 3.

So far, it's been a race to prove which candidate is more conservative, which was Tuesday's theme.

Tiahrt accused Moran of not supporting former President George W. Bush's tax cuts, while Moran hit his opponent on his vote in favor of the Medicare prescription drug benefit.

One of the debate's most contentious exchanges came on the issue of immigration. From the AP:

[Moran] criticized Tiahrt during the debate for past support of proposals to allow some illegal immigrants to obtain driver's licenses and to make it easier for some to pay lower college tuition. Tiahrt said his views shifted as he consulted with Kansans and studied the issue.

... Tiahrt said he not only wants to complete the border fence but wants to stop printing bilingual election ballots, a practice he said encourages people to come to the U.S. illegally. Moran endorsed the new Arizona law - directing local police to check people's immigration status - and derided the Obama administration's challenge as "a silly waste of taxpayer dollars."

But amid that tough talk, Tiahrt acknowledged he once supported proposals on tuition and driver's licenses; Moran's staff noted Tiahrt co-sponsored legislation in 2001 and 2003. Supporters of such policies say they mostly help children who are brought to the U.S. by their parents and stay into young adulthood.

"A long time ago, I thought it was compassionate to not punish the children for the parents' sins, but I've listened to the people of Kansas. I've studied this issue," Tiahrt said. "I've come to the conclusion that we must build the fence, that we must say no to amnesty."

Tiahrt also reminded voters he has the backing of the Tea Party Express and former Alaska Gov. Sarah Palin, who endorsed him last month. Moran has the endorsement of Sen. Jim DeMint (R-S.C.) who will campaign Friday for Moran in Kansas. 

A Survey USA poll from late June gave Moran a 20-point lead.