AP

Thad Cochran

Incumbent Republican Sen. Thad Cochran (R-Mississippi) has improbably survived a grueling primary challenge from Tea Party-aligned insurgent state Sen. Chris McDaniel.

In what was called the nastiest election fight in America, Cochran defeated McDaniel in the state's runoff election Tuesday night to officially become the Republican nominee for Senate, the Associated Press projected.

As of 11:15 p.m. ET, Cochran led McDaniel, 50.8-49.2, with 99% of precincts reporting.

For Cochran, the win marks an unthinkable turnaround from a primary election three weeks ago, in which he lost a plurality of the votes to McDaniel. There was a runoff only because neither candidate got 50% of the vote.

"It's a group effort. It's not a solo. And so we all have a right to be proud of our state tonight," Cochran said in brief celebratory remarks to supporters Tuesday night.

The fundamentals of the runoff were largely expected to favor McDaniel. In recent years, runoff elections have typically favored the insurgent candidates whose supporters are more invigorated to come out for a second vote on an atypical election day. The most recent prominent example came in Texas in 2012, when current Sen. Ted Cruz (R-Texas) knocked off Texas Lt. Gov. David Dewhurst in a runoff.

McDaniel also claimed momentum in the wake of House Majority Leader Eric Cantor's stunning GOP primary loss, saying it boosted his argument that voters across the country wanted change from the status quo in Washington. He had support from grassroots groups like the Club for Growth and Senate Conservatives Fund.

But turnout actually increased in the runoff, a rather unprecedented phenomenon. The last time that happened in a Senate runoff was in 1984 in Oklahoma. More than 363,000 Mississippians cast their vote in the runoff election, a 313,483 in the primary.

Despite the fundamentals and polls showing Cochran at a significant disadvantage, the campaign remained "cautiously optimistic."

" We have out worked them in this run-off. We’ll see if it overcomes their rabid enthusiasm," a source close to the Cochran campaign told Business Insider Tuesday morning.

The Cochran campaign pivoted its strategy during the runoff campaign, traversing across the state to have Cochran boast of the federal money he brought back to benefit the state. He labeled McDaniel "dangerous" and an "extremist."

Cochran also made it a priority to turn out non-traditional primary and runoff voters, including high-profile targeting of black Democratic voters. Cochran was supported by outside establishment groups including the U.S. Chamber of Commerce. Former Mississippi Gov. Haley Barbour was also a powerful backer of Cochran.