It's primary day in eight states! Voters are heading to the polls in Alabama, Mississippi, New Jersey, New Mexico, South Dakota, Iowa, Montana and California. Here are the five biggest things to watch:

1. Will Thad Cochran become the first U.S. senator to fall in a 2014 primary?

Sen. Thad Cochran (R-Miss.) (Joe Ellis/Clarion-Ledger via AP)

The Republican primary for U.S. Senate in Mississippi has been downright nasty, deeply personal and fiercely competitive. The latest polls show a very close race between Sen. Thad Cochran and tea party backed-state Sen. Chris McDaniel. Get to know the name Tom Carey. He's like to finish a distant third behind the two frontrunners, but if Carey pulls enough support (8 percent could do it), he could prevent Cochran or McDaniel from winning a majority of the vote, sending the race to a runoff on June 24. Runoffs are not typically good news for incumbents. Just ask Rep. Ralph Hall (R-Tex.). Polls close at 8 p.m. Eastern time. 

2. Will Joni Ernst top 35 percent in Iowa?

Joni Ernst speaks as former Alaska governor Sarah Palin, left, looks on at a campaign rally on April 27 in West Des Moines, Iowa. (Charlie Neibergall/AP)

Both the tea party and the Republican establishment have lined up squarely behind state Sen. Joni Ernst in the Republican primary for U.S. Senate. Polls show her with a wide lead over businessman Mark Jacobs, a lone wolf who has relied on his personal wealth to stay relevant in the race. But finishing first won't be enough for Ernst. She has to win more than 35 percent of the vote, otherwise party activists will choose the nominee at an unpredictable state convention on June 14. Recent polling shows Ernst hovering right around the magic number. And with a late push from the likes of Mitt Romney and Sen. Marco Rubio (R-Fla.), GOP strategists are hopeful she's picked up enough momentum to clear the magic number. If she doesn't, it's good news for Rep. Bruce Braley, the likely Democratic nominee. Polls close at 10 p.m. Eastern.

3. Will Democrats get embarrassed in California's 31st district again?

Baca getting a high five in 2012 from Golf Channel's Michael Breed. (Dan Curran/We Are Golf )

There's a lot going in in the Golden State, which has an all-party primary system in which everyone running competes on the same ballot. The top two candidates advance to the general election regardless of party affiliation. Democrats know the pain this can cause them in the 31st district, a left-leaning seat they should have no business losing to a Republican. But they did in 2012, when a vote split among their candidates embarrassingly shut them out of second round. There's a chance it happens again today, but not a great one. Once again, the party is split. The Democratic Congressional Campaign Committee backs Redlands Mayor Pete Aguilar; Emily's List supports attorney Eloise Gomez Reyes; and former congressman Joe Baca has a secretive group in his corner. One of the three is a good bet to advance to a showdown against businessman Paul Chabot (R). But if they cancel each other enough, it's remotely possible another Republican sneaks in past them like in 2012. Polls close at 11 p.m. Eastern. 

4. Oh, and there's lots more worth watching in California. 

Republican businessman Neel Kashkari, right, talks with Kenneth Whitaker, 62, at the Loaves and Fishes homeless shelter in Sacramento, Calif. (Rich Pedroncelli/AP)

Former U.S. Treasury aide Neel Kashkari's moderate political profile would probably make him Republicans' best bet in a long-shot bid to unseat Gov. Jerry Brown (D) in the fall. But tea party-aligned Assemblyman Tim Donnelly (R) might be the one who advances. In the race for retiring Democratic Rep. Henry Waxman's seat, there are 16 (!) candidates on the ballot. Meanwhile, an intense race between Democratic Rep. Mike Honda and former U.S. Commerce Department official Ro Khanna is headed to round two, but will Honda finish first today? (Strategists expect that he will.) Polls close at 11 p.m. Eastern. 

5. Open U.S. House primaries are all the rage today. 

Tom MacArthur (R) answers a question in Brick Township, N.J. (Mel Evans/AP)

The seat of retiring Rep. Tom Latham (R-Iowa) is up for grabs in November, and while Democrats have rallied around state Sen. Staci Appel, Republicans are sorting out a six-way primary that could be headed to a convention. In New Jersey, the retirement of Rep. Jon Runyan (R) has opened up another battleground. There, establishment-backed wealthy businessman Tom MacArthur  is trying to get past tea party-backed businessman Steve Lonegan in the GOP primary. Democrats appear to have meddled in the hopes of boosting Lonegan. Burlington County Freeholder Aimee Belgard is the frontrunner in the Democratic race. Finally, it's not an open race, but keep an eye on the battle in Mississippi's 4th district featuring Rep. Steven Palazzo (R) and Democrat-turned-Republican former congressman Gene Taylor, which could head to a runoff. Polls close at and 8 p.m. Eastern in New Jersey and Mississippi and 10 p.m. Eastern in Iowa.