Sen. Jeff MerkleyJeffrey (Jeff) Alan MerkleyThe federal judiciary needs more Latino judges Overnight Defense: Trump decision on Korea summit coming 'next week' | China disinvited from major naval exercise | Senate sends VA reform bill to Trump Senate sends major VA reform bill to Trump's desk MORE (D-Ore.) is facing a sixth potential Republican opponent as he seeks a second term in 2014, with pediatric neurosurgeon Monica Wehby joining a crowded Republican primary field on Tuesday.

Wehby is well-known in Oregon and among medical policy advocates, and has been an outspoken critic of the Affordable Care Act.

She served on the board of the American Medical Association — a position from which she resigned to run for office — and in 2004 unsuccessfully pushed for a ballot measure to limit non-economic damages in medical lawsuits.

In a video announcing her Senate bid, Wehby emphasizes her outsider credentials.

"I'm not a politician. I'm not part of the establishment. I'm a doctor who has devoted my entire career to treating very sick children and to helping families at the most vulnerable times in their lives," she says.

Wehby will compete against five other declared Republican candidates for the chance to challenge Merkley. The most prominent Republicans in the race are state Rep. Jason Conger and businessman Sam Carpenter.

The eventual Republican nominee is expected to face an uphill battle in the 2014 general election. Oregon leans blue and Merkley has about $2.2 million cash on hand for his reelection fight.

Still, while the state isn't a top GOP target, Republicans have competed in the state before and the party hopes it can put Oregon into play. Prior Merkley's election in 2008, the seat was held by two-term Republican Sen. Gordon Smith.