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Editor's note: Julian Zelizer is a professor of history and public affairs at Princeton University. He is the author of "Jimmy Carter" and "Governing America."

(CNN) -- The midterm elections are around the corner. The big question will obviously be what happens to control of the House and Senate. But control of Congress is only one part of the equation. There are a series of issues that will shape the individual races that will tell us a lot about which way American politics is heading.

Can mainstream Republicans take the party back? There are a number of House and Senate primaries where Republicans, organized through business-backed organizations, are trying to seize back control of their party.

Former Ohio Rep. Steven LaTourette and the Main Street Partnership, a group with strong backing from the corporate world, are trying to counteract the power of the tea party, which they believe is damaging the standing of the GOP. "We want our party back," LaTourette explained to the The New York Times.

In the 2nd District of Idaho, Rep. Mike Simpson is facing a strong challenge in the May primaries from Bryan Smith, a tea party Republican who has received support from the Club for Growth. Smith is challenging the eight-term incumbent by depicting him as an embodiment of Washington Republicans who refuse to stand firm for real budget cuts, a legislator who agreed to reopen the federal government even without what conservatives would consider a good budget deal. The Main Street Partnership has been fighting back.

The most visible battle between a mainstream Republican and tea party Republican is taking place in Kentucky, where Senate Minority Leader Mitch McConnell is facing a challenge from Matt Bevin in the primary. In Texas, the controversial right-wing Rep. Steve Stockman is running against Sen. John Cornyn.

Can Democrats take advantage of Republican problems? Despite the fallout from the rollout of the Affordable Care Act, Democrats are hoping that they can take advantage of the turmoil facing the Republicans over the recent year, as the party's approval ratings have plummeted in light of the budget battles and the public's unhappiness with the party's growing extremism.

There are certain must-wins for Democrats if they are to show that they are capable of taking advantage of this moment. In Florida's 13th District, Alex Sink, a well-known and well-respected Democrat, is attempting to win the seat of long-term Republican veteran Bill Young, who recently died, leaving open this highly competitive district. If Democrats can't win this special election on March 11, it will signal trouble.

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Democrats will also be looking for a win in Florida's 2nd District, where Gwen Graham is trying to defeat Rep. Steve Southerland in a test of whether the South has really softened as conservative territory. The Democratic Congressional Campaign Committee has been pouring resources into the district to paint Southerland as a poster child for the House GOP. "Congressman Southerland's reckless plan to repeal the entire Affordable Care Act would mean 614,200 consumers in Florida would be left without health insurance rebates," said one party spokesperson.

What effect will the politics of immigration have? After the 2012 election, many experts predicted that the immigrant vote would continue to flow toward the Democrats while Republicans will pay the electoral price for their obstruction. Now we'll find out if that's correct.

In Colorado's 6th District, Republican Mike Coffman, a former opponent of immigration reform who changed his tune after redistricting brought an infusion of Latinos into his constituency, is struggling to hold on to his seat. Democrats are hoping that the sizable Hispanic population in suburban Denver will demonstrate their opposition to what the House Republicans have been doing by blocking legislation that would offer a path to citizenship.

Which is the more politically potent issue -- opposing Obamacare or supporting the minimum wage? Both parties are putting forth national issues for candidates to run with in their districts and states. Democrats are honing in on the issue of economic inequality, stressing their campaign to raise the minimum wage from $7.25 to over $10 by 2015.

For Democrats, the risk is alienating Democrats from centrist constituencies who believe this will damage the economy. But they hope that the strong support in polls for the minimum wage boosts their candidates. Rep. Steve Israel, chair of the Democratic Congressional Campaign Committee, predicts that there are at least 12 seats where labor-based campaigns to raise the minimum wage will help Democrats. "The refusal to increase the minimum wage is just one of the ways House Republicans have inflicted harm on the economy and hurt people's pocketbooks," Israel said.

For Republicans, the risk is they will be perceived as a party that is stuck in the mud, at a time that millions of Americans are starting to finally see benefits from the ACA rather than simply see the program as something abstract that threatens their families. In Arkansas, Republicans are counting on Sen. Mark Pryor's support for the Affordable Care Act as a useful target for Rep. Tom Cotton to unseat this incumbent. Republicans will do the same in Louisiana, where Sen. Mary Landrieu has a record of statements in support of the health care program.

How much money should we spend on elections -- and where should it come from? This is the question now asked of every congressional and presidential election, but it is one we need to continue asking. On both sides -- the U.S. Chamber of Commerce, Freedom Works, Club for Growth, and America Rising on the right and American Bridge and Priorities USA on the left -- independent organizations are ready to roll in an unprecedented effort.

With each election we have been seeing the cost of campaigning rise and the need for candidates to court donors as more and more urgent. The Kentucky Senate race will probably be the most expensive ever. All of this fuels the power of private money in our political system, undermining trust in government and supporting gridlock.

When the results become clear in November, we will know a lot more about the general tenor of the American electorate. Politicians in both parties will have a better read of the kind of electorate they will be facing as they move into the 2016 presidential election.

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The opinions expressed in this commentary are solely those of Julian Zelizer.