TOPEKA ? Republican Sen. Pat Roberts, facing a tea party challenger as he seeks a fourth term, has launched a new campaign ad ahead of the August primary touting his conservative credentials and service as a Marine.

The 30-second TV spot began running this week in the Wichita and Topeka markets and in the Kansas City metro area, home of Roberts’ primary opponent, Milton Wolf, a radiologist from Leawood making his first run for political office.

The ad aims to highlight differences between the three-term incumbent and Wolf, Roberts’ campaign spokesman Leroy Towns said Thursday.

“It’s meant to reinforce the senator’s longtime conservative credentials,” Towns said. “Pat Roberts has been fighting the conservative Republican fight for a long time.”

The ad features Derek Brunin, a Marine from Rossville who served in Iraq. Brunin says in the commercial that Roberts has opposed higher taxes, increased spending and the federal health care law. The campaign plans to spend at least $150,000 on the ads, but that could rise if the run extends past the initial two weeks.

Wolf spokesman Ben Hartman said the latest Roberts ad is an attempt to portray the senator as being “in the driver’s seat” going forward.

“I think he’s forced to reintroduce himself,” Hartman said. “It suggests that they may have some internal polling that shows his numbers are sliding.”

Wolf’s campaign unsuccessfully challenged Roberts’ status as a legal Kansas resident and eligibility to be on the Aug. 5 ballot. Wolf has tied the senator’s residency to his ownership of a home in Alexandria, Virginia. Roberts is registered to vote in Dodge City at the home of a couple who are longtime political supporters and rent him a bedroom and bathroom. Roberts and his wife also own a duplex in Dodge City but rent it out.

The State Objections Board ruled that Roberts was a state resident and eligible to run for re-election.

Hartman said he expected the campaign would see more made of Roberts’ residency and the amount of time he spends in the state. He said Wolf had a “sizeable ad campaign” planned in the coming weeks.