JACKSON, Mississippi (CNN) -

Polls have closed in Mississippi, where one of the longest serving members of Congress is fighting for his political life.

The primary runoff involving Republican Sen. Thad Cochran is one of the marquee races Tuesday as seven states from the East Coast to the Mountain West hold primaries.

Another top contest involves Democratic Rep. Charlie Rangel of New York, who's also served four decades on Capitol Hill and is trying to avoid being pushed out of office.

Also in the spotlight is a high-profile tea party vs. establishment Republican contest in the GOP Senate primary in Oklahoma.

Democrats may swing GOP contest

In an odd twist, the final results for the Republican runoff in the deeply red state of Mississippi may actually depend on Democrats.

Neither Cochran, nor his tea party challenger, Chris McDaniel, secured 50% of the vote in the Republican primary three weeks ago, which forced the race into a runoff. McDaniel, a state senator, edged out Cochran by less than 1,500 votes.

To clinch the GOP nomination, both sides and their allies have spent a lot of money to gin up more support. Cochran's backers are turning to Democrats, especially African-Americans who make up 37% of the state's population.

Cochran's supporters have been actively reminding voters of the senator's work to secure federal funds for program relied upon by African-Americans, like Head Start and certain medical centers in the state.

It's the kind of message that Republicans barely tout these days, given the renewed focus on fiscal conservatism. But Cochran supporters view Democrats as key to their strategy to knock out McDaniel.

One precinct in a predominantly African-American area showed promising signs for Cochran: Voter turnout had tripled in the area compared to three weeks ago. But the question is whether McDaniel will get a boost, as well.

Mississippi law allows anyone to vote in the runoff, meaning Democrats can go to the polls so long as they didn't vote in the Democratic primary and they don't plan to support their party candidate in the general election.

To make sure Democratic voters weren't voting illegally, conservative groups supportive of McDaniel dispatched volunteers to observe poll workers and whether they're turning away those who already showed up in the Democratic primary.

But that effort raised eyebrows from groups like the NAACP, which sent out their own volunteers to look for any signs of voter intimidation or interference.

Efforts to get-out-the vote among Democrats in the Republican runoff reflect the high stakes locally and nationally in the Cochran-McDaniel showdown, and illustrate the unscripted political realities of particular states that play out on Election Day.

It's unclear who will be crowned victor in this circus of a contest, but it's likely the race will be close.

Rangel's last dance

Win or lose, Rep. Charlie Rangel of New York says this is his last campaign.

The Korean War veteran, who was first elected to the House of Representatives 44 years ago, is trying keep from getting pushed out office by his main primary challenger, state Sen. Adriano Espaillat, who came within around 1,100 votes of ousting Rangel in the Democratic primary two years ago.

Rangel, the former chairman of the tax-writing Ways and Means Committee, was forced to step down from his post in 2010 and later that year he was censured by the House for ethics violations.

Just as damaging for Rangel was the redrawing of New York's 13th Congressional District after the 2010 election, from a Harlem-based, African-American-dominated district to one that now has a Hispanic majority, thanks to shedding parts of Harlem and adding other neighborhoods in northern Manhattan and parts of the Bronx.

Rangel, the "Lion of Harlem" and a founding member of the Congressional Black Caucus, said he didn't put up much of a fight in 2012.

"I didn't have a campaign last time. When he told me he was running, I was in Columbia Presbyterian with a viral infection in my spine."

This time, Rangel said he's ready.

"Well, I don't have a walker. I don't have a spinal injury."