During the last major day of primaries until August, a few of the most prominent themes of the 2014 midterm election cycle will be on display.

In Oklahoma, Rep. James Lankford, running for Senate, will test the weight of a House leadership position amidst widespread anti-establishment sentiments, just two weeks after House Majority Leader Eric Cantor lost his re-election bid.

In Mississippi and New York, meanwhile, two of the longest-serving members of Congress will try to push back against low approval ratings for Congress to hold on to their seats.

Here's a quick guide to what to look for tonight.

Mississippi Senate

For most of his re-election bid, Sen. Thad Cochran, R-Miss., has been the political equivalent of the deceased boss in “Weekend At Bernie's”: propped up by fellow senators and the National Republican Senatorial Committee, but with his campaign lacking its own pulse.

Tuesday, when Mississippi Republicans will decide a run-off between Cochran and his Tea Party challenger, Chris McDaniel, will show if Cochran revived himself just in time.

A nearly-four-decade incumbent, Cochran has raised vastly more money than McDaniel, a first-time Senate candidate, and Cochran’s campaign has spent much of it on a brutal crush of ads in the two weeks preceding the run-off. In one attack ad, Cochran’s campaign played controversial remarks by McDaniel in rapid succession.

Outside groups have also played a large role in the race, which could be the best chance the Tea Party has to pull an upset in a Senate race this election cycle. McDaniel has attracted support from FreedomWorks, the Senate Conservatives Fund and other groups; meanwhile, the Chamber of Commerce has been on the air boosting Cochran with ads starring Mississippi native Brett Favre. The Karl Rove-backed group American Crossroads opted not to defend Cochran in the run-off.

But even with the powerful political forces at his back, Cochran goes into the run-off election Tuesday as an underdog. Special election turnout will be lower than in a normal election, likely drawing staunch activists to the polls.

Oklahoma Senate

There is very little difference on policy between Rep. James Lankford and State Rep. T.W. Shannon, the two leading Republicans vying for their party's nomination in the Oklahoma Senate special election.

Both men are very conservative — but as the fifth-ranking member of House Republican leadership, Lankford has found himself on the receiving end of attacks from Tea Party groups, who have derided him as too “establishment.”

A recent poll found Lankford leading Shannon with 43 percent to 35 percent, but if neither candidate wins 50 percent Tuesday, the race will be decided with a run-off in August. The election is being held to replace Sen. Tom Coburn, R-Okla., who will retire at the end of this year.

New York's 13th Congressional District

Rep. Charlie Rangel, D-N.Y., has served in Congress for more than four decades, but that streak could be at an end.

Rangel will face off Tuesday in a primary against state Sen. Adriano Espaillat, who came close to besting Rangel two years ago. This time, Espaillat will likely benefit further from a Hispanic population that has ballooned in Harlem and from Rangel’s diminished brand.

In an election cycle characterized by a deep, pervasive anti-Congress sentiment among voters, longtime incumbents of both parties, such as Rangel, appear to be particularly endangered.

Rep. Ralph Hall, R-Texas, the longest serving member of Congress, earlier this year lost in a primary race. In Mississippi, Cochran faces a similar fate. Cantor, though not a longtime incumbent, likely suffered from such an anti-establishment strain when he lost his primary race to unknown upstart Dave Brat.