Voters go to the polls today in Indiana, North Carolina and Ohio in one of the busiest days on the 2010 primary calendar. The tri-state voting also will initiate a five-week burst of election activity that will see 23 states hold primaries and three states hold special House elections.

Today’s returns will provide the clearest sign to date of the mood of the electorate six months before the midterm elections as well as test the clout of the political establishment and the national parties that have promoted preferred candidates in some contested primaries.

Here’s a look at key primaries to watch:

Indiana

Senate

Out of the Indiana political limelight for more than a decade, former Sen. Dan Coats (R) showed some rust in his comeback campaign for the seat he used to hold. But he seems to have weathered the storm — at least in the Republican primary — and GOP officials think he will outpoll former Rep. John Hostettler and state Sen. Marlin Stutzman, who are running as stronger bearers of the conservative banner. Democrats will point to an underwhelming Coats win as a sign of weakness among conservative base voters, and they’re happy with their presumptive nominee, Rep. Brad Ellsworth, who will be formally nominated later this month.

House

2nd district

GOP officials are confident that state Rep. Jackie Walorski will convincingly win a four-candidate primary and square off against two-term Rep. Joe Donnelly (D).

3rd district

Rep. Mark Souder, who has sometimes underperformed in his strongly Republican-leaning, Fort Wayne-based district, is vulnerable to a primary challenge from Bob Thomas, a wealthy car dealer who has attacked the Congressman for reneging on a term-limits pledge and for his 2008 vote to stabilize the financial markets. Phil Troyer, a lawyer and former Congressional aide, should also peel off some of the anti-Souder vote. So the Congressman may win with a plurality.

4th district

The GOP primary in this rock-ribbed Republican district will essentially determine a successor for retiring Rep. Steve Buyer (R). The 13-candidate race is headlined by Indiana Secretary of State Todd Rokita and state Sen. Brandt Hershman, who also is a longtime Buyer aide.

5th district

Rep. Dan Burton (R), who was nearly defeated for renomination in the 2008 primary, probably would be a goner if he had one opponent instead of six in his primary. Burton almost certainly will win less than a majority of the vote, but most political observers think the anti-Burton vote will be divided such that the Congressman will eke out a win. Burton’s best-funded challengers are former state Rep. Luke Messer and state Rep. Mike Murphy.

8th district

Republican officials expect heart surgeon Larry Bucshon to top an eight-candidate primary for the Evansville-based seat Ellsworth is giving up. The Democratic nominee will be state Rep. Trent Van Haaften.

9th district

The question for GOP voters in this competitive southeastern district is whether they will go with former Rep. Mike Sodrel as their nominee for the fifth consecutive election or if they opt for a fresh face to oppose Rep. Baron Hill (D). Sodrel’s most threatening challenger appears to be Todd Young, a lawyer who has been campaigning for the seat for more than a year.

North Carolina

Senate

Though a half-dozen Democrats are vying for the nomination and the right to take on Sen. Richard Burr (R), the race has come down to whether state Secretary of State Elaine Marshall (D) can hit the 40 percent threshold that is required to avoid a June runoff. Former state Sen. Cal Cunningham is fighting to make that runoff happen. National Democratic Party officials recruited him last year over Marshall.

Cunningham has had a larger media presence in the Tar Heel State and has been steadily gaining on Marshall since going on the air in early April. Marshall’s media program just got off the ground last week in large part due to her weaker fundraising. But Marshall’s institutional support, stemming from her four terms as secretary of state, may see her through.

House

8th district

A half-dozen Republicans are vying for the chance to take on freshman Rep. Larry Kissell (D), and a June 22 runoff seems inevitable. Millionaire Tim D’Annunzio, who has spent about $1 million of his own money, seems likely to have bought himself a place in the runoff. But the party establishment has made it clear that it believes the outspoken and controversial D’Annunzio is a weak general election candidate. Former sports broadcaster Harold Johnson, who has loaned his campaign nearly a quarter of a million dollars, will likely face D’Annunzio in the GOP runoff. Look for the establishment to move to support Johnson even more then.

10th district

Rep. Patrick McHenry (R) is facing a pair of well-funded challengers who have been running vigorous anti-Washington campaigns against the third-term Congressman: dentist and Iredell County Commissioner Scott Keadle, who loaned his campaign nearly a half-million dollars, and entrepreneur Vance Patterson, who loaned his campaign a quarter-million dollars.

11th district

Half a dozen Republicans are competing in another primary that should go to a runoff before the GOP determines how serious a challenge it will mount against second-term Rep. Heath Shuler (D).

Jeff Miller, who founded a nonprofit group to fly World War II veterans to Washington, D.C., to tour the World War II Memorial and other sites, seems likely to make that runoff. Miller was recruited into the race by national party officials.

Ophthalmologist Dan Eichenbaum, who has put more than $100,000 of his own money into the contest, and Hendersonville Mayor Greg Newman, who entered the race with perhaps the best name identification, are the two most likely candidates to face Miller in the runoff.

Ohio

Senate

A robust television advertising campaign and support from the Democratic establishment have helped install Lt. Gov Lee Fisher as the favorite over Ohio Secretary of State Jennifer Brunner. The bad news for Fisher: Former Rep. Rob Portman is unopposed in the Republican primary and has been stockpiling money in a campaign treasury that had nearly $7 million more than Fisher’s as of mid-April.

House

2nd district

Rep. Jean Schmidt has had a shaky hold on her dependably Republican district in and around Cincinnati, but she should beat three primary opponents and face the winner of a three-candidate Democratic primary in which party officials are backing Surya Yalamanchili, who once appeared on “The Apprentice.”

16th district

Jim Renacci, a businessman and former small-town mayor, is the preferred candidate of the National Republican Congressional Committee in a four-candidate primary. Matt Miller, a former county commissioner, is waging a third bid for the seat after taking more than 40 percent of the vote in each of the past two Republican primaries, but GOP officials think Renacci will win convincingly and then give Rep. John Boccieri (D) a competitive challenge in November.

18th district

State Sen. Bob Gibbs, the NRCC’s preferred candidate to oppose Rep. Zack Space (D), has been trying to build his profile in an expansive rural district where he’s not well-known. Three of the other seven Republicans seeking the seat also ran two years ago.