Sen. Lamar Alexander, center, and wife Honey, left, talk with Grayce Cabage, who has known Alexander all his life. (AP Photo/The Daily Times, Tom Sherlin)

On Thursday, Sen. Lamar Alexander (R-Tenn.) can end a streak Senate Republicans would love to retire.

Alexander is the last of six Republican senators who drew promising primary challengers this year claiming the tea party mantle. If he wins, it will mark the first time since 2008 that conservative insurgents have failed to dislodge a single Republican senator during the primary season.

The series of establishment candidate wins have bolstered Republican hopes of seizing the Senate majority in a year when they have no margin for error. Being stuck with flawed tea party nominees in Kentucky or Mississippi would have imperiled the GOP's quest to gain the six seats they need to control the chamber in 2015.

It seems that after seeing some longtime colleagues get picked off in two consecutive elections, Republican senators may have finally found the formula to keep primary competitors from dislodging them: Take tea party challengers seriously. And take them on early.

"Incumbents like Dick Lugar didn't see the challenge coming until very late," said former Mississippi governor Haley Barbour (R), referencing the Indiana senator who lost in 2012. "The senators this year were more on notice that this was coming."

Alexander -- who polls show is poised to defeat state Rep. Joe Carr, a tea party stalwart and an immigration hardliner -- kicked off his campaign back in December of 2012 with the support of Tennessee’s Republican governor and every Republican member of the state's congressional delegation.

Incumbent odds have also been boosted by the fact that the tea party recruited weaker candidates than in past elections. Carr, for instance, failed to catch fire for most of the race. Only near the end of the campaign did he earn the endorsements of Sarah Palin and conservative radio host Laura Ingraham. In the final days, Ingraham recorded a robocall that she said went out to 300,000 Republican households -- but even she acknowledged the challenge Carr faces.

"It’s difficult to unseat any incumbent, but Carr has built a pretty good ground game," said Ingraham.

After the shocking loss of Rep. Eric Cantor (R-Va.) in June, no incumbent seems safe -- even if, by almost every measure, Alexander appears likely to win comfortably.

Still, even if he falls short, it won't be enough for the tea party to salvage a 2014 that's been full of disappointment for the movement.

Sen. Pat Roberts's win in Kansas on Tuesday was yet another lost opportunity for frustrated conservatives. Sen. John Cornyn (R-Tex.), a member of the GOP leadership, easily outpaced his chief tea party challenger in March, as did Senate Minority Leader Mitch McConnell (R-Ky.) in May. Sen. Thad Cochran (R-Miss.), who, as with Roberts, was questioned about residency issues and his long tenure in Congress, had a tumultuous run but won a runoff election in late June. Sen. Lindsey Graham (R-S.C.) also won his primary that month.

Each of those unsuccessful Senate primary challengers believed they could use the anti-Washington sentiment evident among Republican base voters and their rival’s lengthy voting record to win, but ultimately found it difficult to build a majority GOP coalition. It's a marked contrast with the conservative experience during the 2010 election cycle, when a group of hard charging tea party candidates, such as Marco Rubio (Fla.) and Mike Lee (Utah), emerged victorious in primaries, with Lee ousting Sen. Robert F. Bennett, a longtime incumbent.

But while Rubio and Paul were blue chip recruits, this year's class featured more than its fair share of flawed candidates.

Physician Milton Wolf, who lost to Roberts, cracked jokes about X-rays of gunshot victims on Facebook. Businessman Matt Bevin, who was blown out by McConnell, attended a pro-cockfighting rally and once praised the TARP program he now condemns. Even Chris McDaniel, the darling of national tea party groups, has risked his reputation by refusing to concede more than a month after he lost to Cochran.

This year, the tea party’s high profile wins came in House races, where they defeated Cantor and Rep. Ralph Hall, a 91-year-old Texan, in GOP primaries. Conservatives also scored a win in the open U.S. Senate race in Nebraska, with the victory of Ben Sasse -- though Sasse's occasional positions at odds with the political right, including past support for the Medicare Part D drug benefit, has led some to question how much of a tea party candidate he really is.

Conservatives say that wins and losses aren't the only way to gauge success. They take pride in nudging Republican senators like Cornyn -- who stymied the Obama agenda at every turn in 2013, even refusing to confirm John Kerry as secretary of state -- to the right.

"What I find to be the greatest irony of them all is everybody has one thing in common -- everybody is running as a conservative," said L. Brent Bozell III, the chairman of ForAmerica, a conservative advocacy group.

But while Senate Republicans have improved their odds in primary races, they have not extinguished the intra-party squabbling of past years. If anything, it's intensified.

Brian Walsh, a former adviser to the National Republican Senatorial Committee, blamed national tea party groups like the Senate Conservatives Fund and the anti-tax Club for Growth for financing efforts to unseat Republicans.

"The only real impact these primaries have is that D.C. conservative groups will have wasted literally millions of dollars attacking Republicans that now can't be used against Harry Reid and the Democrats," said Walsh.

But Bozell thinks the GOP establishment's intensified efforts to bludgeon challengers are what's damaging the party.

"They have thrown the kitchen sink against their own base to win," he said. "This is civil war in the GOP."

After two straight bruising presidential nominating contests that forced the eventual nominee to the right, that could be a problem for the GOP heading toward 2016. Republicans are hopeful a more compact primary calendar and fewer debates will spare the party the infighting that plagued the eventual nominee in both cycles. But lingering tensions between tea party loyalists and elected Republican leaders could be a wild card.

"It's very clear that no one will be able to win the nomination in 2016 coming from only one portion of the Republican Party's center-right coalition," said GOP pollster Whit Ayres. "By definition, whoever wins is going to have to stitch together a coalition of people who identify with disparate parts of the party."