It’s the biggest primary day of the year, with voters in a dozen states going to the polls.

But you’d never know it from the media coverage and political buzz. All we’re hearing about is how another incumbent Senator — Arkansas Democrat Blanche Lincoln — could lose Tuesday. And how we’ll learn Tuesday night who Senate Majority Leader Harry Reid’s (D-Nev.) general election challenger is going to be. And how Tuesday’s voting concludes two statewide Republican primaries in California that cost as much as producing your average Hollywood movie. And about the sex allegations and ethnic slurs being thrown around in the South Carolina Republican gubernatorial primary.

Yet with so many states voting, there are also many important House races on the ballot — and they’re being overlooked. So here’s a primer on some of the key House contests.

Arkansas

1st district Democratic runoff

In one of two Razorback State districts in danger of flipping to the GOP in November, Chad Causey, the former chief of staff to retiring Rep. Marion Berry (D), is bidding to replace his boss. Causey enjoys the support of Berry, former President Bill Clinton and several important local officials, and he may have momentum on his side.

But former state Sen. Tim Wooldridge finished first in the primary two weeks ago and is not to be discounted. The winner faces agricultural broadcaster Rick Crawford (R) in November.

2nd district Democratic runoff

State Sen. Joyce Elliott and state Speaker Robbie Wills are competing in the race to replace retiring Rep. Vic Snyder (D). The runoff has turned nasty in the final days, with racial overtones, and many Democratic leaders are whispering that Wills, who is white, will make a stronger candidate in November than Elliott, who is black. The Republican nominee, former U.S. Attorney Tim Griffin, will be formidable.

3rd district Republican runoff

The winner of the contest between state Sen. Cecile Bledsoe and Rogers Mayor Steve Womack will be a shoo-in in November to replace Rep. John Boozman, the Republican Senate nominee.

California

11th district Republican primary

Republicans would like to think that sophomore Rep. Jerry McNerney (D) is vulnerable, but the district is changing, and it isn’t clear yet how strong the GOP nominee will be. The contenders are former U.S. marshal Tony Amador, autism public health advocate Elizabeth Emken, winegrape farmer Brad Goehring and attorney David Harmer, the son of a former lieutenant governor who was the Republican nominee in a special election in an adjoining district last year.

19th district Republican primary

With Rep. George Radanovich (R) retiring, the primary to replace him has become a knock-down, drag-out fight — and several Republican Members could wind up with egg on their faces.

Initially the primary was seen as a battle between state Sen. Jeff Denham, who is Radanovich’s choice, and ex-Rep. Richard Pombo, who represented the 11th district until 2006 and has been endorsed by many of his ex-colleagues.

But as Denham and Pombo and their allies have fired away at each other, an opening has been created for former Fresno Mayor Jim Patterson, who looks like he may win. Fresno City Councilman Larry Westerlund is also in the mix.

36th district Democratic primary

Running from the left in 2006, teacher Marcy Winograd took 38 percent of the vote against Rep. Jane Harman. This time she’s trying again. She probably won’t win, but on a day when the left may knock off a sitting Senator in Arkansas, it will be interesting to see how well progressives do in this Los Angeles-area district.

44th district Republican primary

Rep. Ken Calvert was almost upset by a politically unknown Democrat in November 2008. Now, shrewd Republican insiders think there’s at least a chance that he’ll be upset in Tuesday’s primary by another political unknown — Realtor Chris Riggs.

Georgia

9th district special election runoff

The winner in Tuesday’s runoff between ex-state Rep. Tom Graves and ex-state Sen. Lee Hawkins will replace former Rep. Nathan Deal (R), who resigned to run for governor. Graves, who finished well ahead of Hawkins in the first round of voting, is favored to win. Whoever loses will remain on the July 20 Republican primary ballot for a full term, along with several other candidates.

Iowa

3rd district Republican primary

By most measures, state Sen. Brad Zaun should be the frontrunner in the primary to take on potentially vulnerable Rep. Leonard Boswell (D) in the fall. But national GOP dynamics are volatile enough that being the favorite of party leaders isn’t a guarantee of success. Former college wrestling coach Jim Gibbons is among the other Republicans worth watching.

New Jersey

3rd district Republican primary

In a competitive district, pro football player Jon Runyan is the favorite of party leaders in the race to challenge freshman Rep. John Adler (D). But conservative Justin Murphy has made enough noise in the primary to bear some watching.

South Carolina

1st district Republican primary

The crowded race to replace retiring Rep. Henry Brown (R) features two political scions — attorney Carroll Campbell III, son of the late Gov. Carroll Campbell, and Charleston County Commissioner Paul Thurmond, son of the late Sen. Strom Thurmond.

But state Rep. Tim Scott, the only African-American Republican in the Legislature, could trump them all. A June 22 runoff seems likely.

3rd district Republican primary

Six Republicans are vying for the right to replace Rep. Gresham Barrett (R), who is running for governor. The race features two state legislators, a minister and a disciple of Rep. Ron Paul (R-Texas). Who winds up in the inevitable runoff is anybody’s guess.

4th district Republican primary

Rep. Bob Inglis, whose record would be considered conservative most places, is under attack from the right, and a quartet of Republicans is taking him on. Former federal prosecutor Trey Gowdy seems like the strongest challenger, though state Sen. David Thomas cannot be discounted. If Inglis cannot top 50 percent Tuesday, he could be in trouble in the June 22 runoff. Stay tuned.

South Dakota

At-large Republican primary

This is one of the most underrated primaries in the country, and the winner should be a serious threat to Rep. Stephanie Herseth Sandlin (D) in November. State Reps. Blake Curd and Kristi Noem and South Dakota Secretary of State Chris Nelson are the leading contenders.

Virginia

2nd district Republican primary

Wealthy auto dealer Scott Rigell is the consensus favorite of party leaders to take on vulnerable freshman Rep. Glenn Nye (D) in November. But the Republican electorate is volatile enough that Rigell’s victory is no sure thing.

5th district Republican primary

State Sen. Robert Hurt is the favorite in the primary to challenge freshman Rep. Tom Perriello (D). But conservative activists grumble that he’s not conservative enough. That may not be enough to deny him the nomination, but it will make things interesting Tuesday night — and through Election Day.

11th district Republican primary

Businessman Keith Fimian is seeking a rematch with freshman Rep. Gerry Connolly (D), who is probably secure in his suburban district. But Fimian faces a tough primary battle with Fairfax County Supervisor Pat Herrity.