ST. LOUIS • The Missouri Republican Party is looking to gain some traction on an issue that is causing fresh headaches for the Obama administration and its allies.

The party began airing a radio ad on Thursday criticizing Democratic U.S. Sen. Claire McCaskill for her support of a requirement that employers, including religious institutions, offer health-care plans that cover birth control and contraception services.

The ad accuses McCaskill, herself a Catholic, of  "regulating the Catholic church."

While the state GOP is mining an issue that has the potential to resonate on both sides of the aisle, being at odds with the ecclesiastical leadership is not an unfamiliar position for McCaskill — or necessarily a political liability.

McCaskill unseated incumbent U.S. Sen. Jim Talent in 2006 with the help of a poignant late October TV ad from actor Michael J. Fox which highlighted a position that runs counter to church beliefs.

Fox, who suffers from Parkinson's disease, was offering support for McCaskill's stance in favor of expanding stem cell research.

But it's the same support for broadening stem cell research that a year later contributed to the archbishop revoking an invitation for McCaskill to speak at her daughter's school, St. Joseph's Academy in Frontenac.

In 1999, when McCaskill was in her first year as state auditor, she was set to speak at Chaminade College Preparatory School when the invitation was also withdrawn because of her stance on abortion.

The thorny contraception issue may lead to more tension between McCaskill and church leadership.

Last week, St. Louis Archbishop Robert Carlson wrote a pastoral letter describing the requirement as "a heavy blow" to American Catholics.

McCaskill has been steadfast in her support of the requirement, saying she is against "barriers" for contraception.

"As someone who believes very much that we should be preventing aboritons, I think we should try very hard to give women universal access to birth control without going into their pockets," McCaskill said earlier.