Voters head to the polls in four states today, with Connecticut, Florida, Minnesota and Wisconsin holding congressional primaries.

Connecticut House Speaker and Democratic congressional candidate Chris Donovan is favored to win his primary today despite the arrests of two top aides. (AP Photo/Jessica Hill, File)

1. The most expensive congressional primary in the country

That would be Connecticut’s 5th district, where seven candidates have raised at least $600,000 and five have raised more than $1 million. A total of nearly $10 million has already been raised just to decide each party’s nominee.

The most interesting subplot is on the Democratic side, where state House Speaker Chris Donovan remains the favorite despite the fact that his campaign manager and top fundraiser have both been arrested and charged with corruption. Organized labor and progressive groups remain firmly behind Donovan, who has not been implicated in the wrongdoing and has won the state party’s endorsement as well.

His chief opponent seemed to be former state representative Elizabeth Esty, but a third candidate, 30-year-old Dan Roberti, has benefited from hundreds of thousands of dollars in super PAC ads and got the endorsement Monday of Bill Clinton, who has ties to Roberti’s father, influential Washington lobbyist and fundraiser Vincent Roberti.

On the GOP side, four candidates are well-funded, but party-backed state Sen. Andrew Roraback and businesswoman Lisa Wilson-Foley appear to be the frontrunners.

This is a Democratic-leaning seat, but it was held for two decades by GOP congresswoman Nancy Johnson. And if Donovan emerges and continues to suffer beneath his aides’ scandal, maybe that opens the door to Republicans in the state’s most GOP-friendly district.

2. Watch the margins in Senate races

There are four states holding Senate primaries today, but with the exception of the Wisconsin GOP Senate primary, we pretty much know who’s going to win. (For more on Wisconsin, the day’s marquee race, see Sean Sullivan’s piece from this morning.)

In Minnesota, Sen. Amy Klobuchar (D) will face state Rep. Kurt Bills; in Connecticut, Rep. Chris Murphy (D) and former WWE CEO Linda McMahon (R) are heavy favorites in contested primaries; and in Florida, Rep. Connie Mack (R) is expected to easily dispatch a primary field that includes former congressman Dave Weldon for the right to face Sen. Bill Nelson (D).

The latter two states are the only ones that are considered potentially competitive in the general election, though in Connecticut, that may be a stretch.

But keep an eye on how big both McMahon and Mack win for an idea about how strong they are headed into the fall. And the same goes for Murphy, who faces a statewide official in former Connecticut secretary of state Susan Bysiewicz. All three should win big, but if they can get 60 percent of two-thirds of the vote, that really says something.

Mack’s race is the closest to becoming competitive in the general election, with Republicans feeling better and better about giving Nelson a run for his money.

3. The old leave-Congress-for-three-decades-and-then-make-a-comeback story

Former congressman Rick Nolan (D) is attempting a political comeback more than three decades after retiring from Congress. And today, he faces former state senator Tarryl Clark for the right to challenge freshman Rep. Chip Cravaack (R-Minn.) in a Democratic-leaning district.

Nolan won the state party’s endorsement and has even got its financial backing, but Clark has raised far more money and secured Clinton’s endorsement. Nolan remains the favorite, though, in a state where the state party’s backing matters – a lot.

The winner will be running in one of Democrats’ top-targeted districts in northern Minnesota (one of the few places in this country that can rightly be labeled “God’s country”).

4. Mica vs. Adams

At least one incumbent will lose reelection today, with Reps. John Mica and Sandy Adams facing off in Florida’s new 7th district.

Mica appears to be the favorite heading into election night. Adams got the backing of some tea party groups and Sarah Palin, but never became a cause celebre and has suffered financially because of it. She got basically no outside groups to spend money on her, and Mica has raised and spent twice as much as she has.

(For more on this race, see Alex Isenstadt’s piece in Politico.)

Elsewhere in Florida, Reps. Cliff Stearns (R) and Frederica Wilson (D) both face primary challengers of some substance, but Stearns’s primary is crowded (which is good for him) so he should be fine, and Wilson has landed the endorsement of President Obama in her racially tinged matchup with Haitian-American candidate Rudy Moise.

Wilson is African-American but represents the most heavily Haitian-American district in the country. She beat Moise 35 percent to 16 percent in a crowded field in 2010, but this year, it’s one on one.

5. Elsewhere in Florida…

A couple second-tier races for either side could feature some upsets.

In freshman Rep. Steve Southerland’s (R-Fla.) district, former state senator Al Lawson (D) is expected to defeat state Rep. Leonard Bembry, a Blue Dog whom Democrats initially had some hopes for.

And in the newly created 9th district, repeat candidate Todd Long appears likely to defeat Osceola County Commission Chairman John Quinones in the GOP primary for the right to face former congressman Alan (“Die Quickly”) Grayson (D).

Grayson and Southerland will both be favored to return to Congress regardless, but that may be even more the case now.