Sen. Lamar Alexander looks like he'll be the latest Republican incumbent to successfully fend off a Tea Party challenger.

The two-term senator appears headed to victory Thursday over challenger Joe Carr in the Republican primary in Tennessee.

A former governor and one-time presidential candidate, Alexander had more experience and more cash than Carr and he was running in a state that tends to favor pragmatic politicians. He has faced some criticism for supporting the Senate's comprehensive immigration reform bill.

Alexander was also helped by the way the Volunteer State holds primaries. Under its open primary system, a candidate only needs a plurality of votes, not a majority and there are no runoffs.

At least some political operatives following Thursday’s contests in Tennessee expect voter participation in the Democratic and Republican primaries to be the smallest and largest on record, respectively, in the history, when judged against comparable years where the governor and local county posts are up for election.

For a state that used to be a Democratic bulwark, this could be viewed as the culmination of a major political sea change.

Here are the other factors buoying Alexander:

It’s a multi-candidate race. Carr, a state legislator, is Alexander’s main challenger and the candidate who has garnered the support of national Tea Party activists. But physician George Flinn is a known political figure and has sunk some of his own money into the race. He’ll pull some moderate Republican votes in Memphis, in the southwestern corner of the state, who would otherwise back Alexander. But combined with some of the other GOP candidates who are on Thursday’s ballot, Flinn actually helps Alexander by dividing up the anti-Alexander vote, thereby boosting the incumbent’s chances of winning a plurality.

East Tennessee authenticity. Alexander hails from Maryville, in East Tennessee, and to borrow a phrase from country music: He never went Nashville. In Volunteer State politics, this is a big deal. About 40 percent of Thursday’s vote is expected to come from East Tennessee counties, a Republican bastion since 1860. These counties opposed secession, provided troops to the Union during the Civil War and were a big reason why Tennessee initially stuck with the Union at the war's outset before eventually seceding. In this part of Tennessee, voting in a Republican primary is a matter of civic duty, not ideology. Voters here have historically despised being told what to do by the financial political interests in Memphis and Nashville, the state capital and Carr’s home base. Alexander (and Republicans like Sen. Bob Corker and Gov. Bill Haslam) has profited from this dynamic time and again, including during his successful gubernatorial campaigns.

“This non-ideological cast, meshed with the provincialism that binds Appalachian people together against flat-landers, helps Lamar get more votes than he would otherwise be entitled to,” said Republican observer of Tennessee politics who is not affiliated with Alexander’s campaign.

Heavy turnout. This year, Primary Day for federal offices coincides with the general election for local offices like county sheriff, county executive and school board. That is expected to drive voter turnout and ensure that this isn’t a typical, lightly attended mid-summer primary that could increase the influence of the most ideologically driven Republicans or independent-leaning conservatives.

The basics. In addition to factors unique to Tennessee GOP primaries, Alexander has the basics covered. He’s raised far more money than Carr, has the more effectively run political operation and is still a great campaigner who hasn’t lost touch, culturally, with Volunteer State voters. It's possible that Alexander could lose the counties in the Nashville media market and finish with less than 50 percent of the vote. That could matter more in future primaries, because Nashville is rising in influence in Tennessee GOP primaries. But for now it would not be a deal-breaker.

“Lamar is an outstanding campaigner,” the GOP source said. “He has an ear for the music of a political campaign. He loves Tennessee and its culture and Tennesseans appreciate that, even if they disagree with him ideologically on a few things.”