On Tuesday, Texans will go to the polls to choose their House and Senate nominees for each party. There’s an interesting contest on the Republican side, where favorite David Dewhurst is challenged in his bid for the Senate by a tea party candidate, but either way Texas will be reliably Republican in the fall, given that the Democrats rarely win even with a good candidate — and they’ve failed to recruit one this time around.

Nationally, though, this month has seen the battle for the Senate expand, with an ever-increasing number of seats in play. Here’s the overview. Remember, in the Senate every seat counts, because every senator has the ability to influence any bill; it’s not like the House, where all that really matters most of the time is which party has at least 218 members.

Right now, the Democrats have a 53-47 advantage in the Senate (yes, it makes sense to count both independents who caucus with the Dems that way). The Democrats, however, have a whopping 23 seats to defend – with seven retirements – while the Republicans have 10, with only three retirements. That’s a terrific playing field for the GOP.

However, on balance, recruiting seems to have favored the Democrats, as has the upset loss by incumbent and general-election-lock Dick Lugar in Indiana. It’s still early, and a few competitive primaries could still dramatically shift a few contests, but overall, the outcome remains wide open. Here’s Charlie Cook’s current estimate of where the races sit; here’s Stuart Rothenberg’s. By my count, there are enough competitive races that all outcomes from a three- or four-seat Democratic pickup to a nine- or 10-seat GOP gain are entirely plausible; in other words, either party could wind up with as many as 57 seats.

What’s more, Senate races tend to be somewhat less closely linked to the rest of the ballot than are House elections. It’s unlikely that Democrats will gain seats if Obama loses, or that Republicans will sweep everything in play if Romney goes down, but it’s certainly very possible that we’ll wind up with one party in the White House and the other with a Senate majority — in either direction.

After the Texas primary Tuesday, here’s what to watch for in June. New Mexico Democrats have a contested Senate primary on June 5, with the winner perhaps a tiny favorite to hold the seat for the Democrats; Rep. Martin Heinrich seems to be in solid shape there.

On the 12th, North Dakota Republicans will select a Senate candidate who will be either solidly (if it’s Rep. Rick Berg) or somewhat (if he’s defeated) favored. And then on the 26th, Utah Republicans will decide the fate of veteran Sen. Orrin Hatch; they’ll retain the seat either way.

There is tons of hype about the presidential election, and it certainly is the most important election this year. But it’s not the only one that matters. Congressional races are underappreciated cycle after cycle, despite the obvious importance of Congress once a president tries to get anything done. I recommend keeping some of your attention — and if you’re a volunteer or a donor, your resources — focused on these important contests.