US. Senator Mark Kirk (R-Ill.), elected in 2010 after five terms in the House of Representatives, may be facing a challenge from within his own party.

James Marter, a 52-year-old computer analyst with no political experience, is formally challenging the incumbent. In a campaign leaflet, Marter says he is running because:

"Kirk is one of the most liberal Republicans in the Senate. He is a full-blown social liberal who supports taxpayer-funding for abortions and the entire gay marriage agenda, including judicial activism and provisions that undermine religious liberty."

Marter's developing campaign website also reports that his disagreements with Kirk extend beyond social issues:

"He is a consistent supporter of gun control with an 'F' rating by the NRA, and has a history of opposing school choice, and has opposed almost every effort to significantly cut spending or balance the budget, and finally he is the only sitting Republican senator to have voted for Cap and Trade energy taxes."

Already known as a social liberal among Republicans, Kirk voted to continue funding Planned Parenthood even after the controversial undercover videos appeared over the summer. Kirk also issued a press release offering his approval of the Supreme Court's same-sex marriage ruling in June.Kirk's deviations led to his

censure by the Aurora Township Republican Organization. His name will not appear on any of the organization's campaign literature. Committee Secretary Jennifer Nevins said, "That will affect the 60,000 voters in our township."

James Marter's challenge may demonstrate how vulnerable Kirk is within his party. If higher profile names such as former U.S. Reps. Joe Walsh and Bobby Schilling decline to enter, Marter could be a candidate for disaffected conservatives and party regulars to rally around.

The Democratic opponent in the 2016 Senate race is likely to be U.S. Rep. Tammy Duckworth of the 8th congressional district. A July Public Policy survey showed Duckworth defeating Kirk 42-36. Another poll in September showed her ahead 45-41.

Kirk may prove to be one of the more vulnerable Republicans in 2016. He won in the Republican-dominated 2010 midterm elections by defeating Democrat Alexi Giannoulias. However, his victory was by less than 2 percentage points in which he received under 50% of the vote.