Thad Cochran, the six-term senior senator from Mississippi, bucked widely held assumptions that he would retire by announcing on Friday that he will seek another term. His announcement sets up a primary showdown between Cochran, who enjoys the support of establishment Republicans, and Chris McDaniel, a young state legislator aligned with the Tea Party.

This will be the most difficult race of Cochran's long career. The veteran lawmaker is used to trouncing his Democratic opponents. His closest race was in 1984 against William Winter. Cochran still won by 22 points. But McDaniel has already lined up an impressive list of backers, including the Club for Growth, and Cochran has yet to begin seriously fundraising. As of Oct. 1, he only had about $800,000 in the bank, leading many party insiders to think Cochran was going to announce his retirement.

Cochran has spent the better part of the last four decades making a name for himself as a lawmaker who brings a large amount of federal dollars back to his home state. In 2010, he steered at least $500 million to Mississippi. Like most Republicans, he voted for the Iraq War resolution and opposed the Affordable Care Act. He is staunchly anti-abortion and pro-military.

Despite this, he is still considered a moderate by southern Republican standards, and some of his votes have rankled conservative purists. He voted for the Democrats' 2010 jobs bill and the New START nuclear agreement with Russia. He also supported Chuck Hagel's nomination for Defense secretary.

In today's hyper-polarized political environment, that makes Cochran vulnerable. The American Conservative Union gave him a rating of 52 percent in 2012, which was down 18 points from the year before. To survive the primary, Cochran will surely have to veer right, especially on fiscal issues.

And now, what was once considered his greatest strength may be his undoing. McDaniel has already zeroed in on Cochran's use of earmarks to bring money back to Mississippi.

"The national debt is the greatest moral crisis of this generation," McDaniel said in October. "So, let's go forth from this place making it perfectly clear that the era of big spending is over. The age of appropriations must end."

Cocrhan, who turns 76 today, can surely expect more of that sort of rhetoric from his 41-year-old opponent between now and the June 3 primary.