College basketball and the dawn of spring are hardly the only highlights of March. Congressional primaries are now in full swing, and the outcomes of key races could decide control of the Senate in November.

From now until Labor Day, the election calendar is sprinkled with competitive Senate primaries. The question is: How much will those contests ultimately affect each party’s majority prospects?

Democrats are fighting to hold their majority as Republicans angle to net the four seats necessary to regain the majority. The numbers are certainly there for the GOP — Democrats are defending 23 seats to just 10 for the GOP, and Democrats have more than their fair share of competitive open seats. But Democrats say their vulnerabilities will be mitigated by potentially destructive GOP fights.

“A number of divisive primaries on the Republican side are likely to positively improve our position in the general, particularly in states like Missouri, Wisconsin and Arizona,” said Guy Cecil, executive director of the Democratic Senatorial Campaign Committee.

All three states hold their primaries in August, leaving less than three months to focus on the general. Another top GOP primary in Nebraska takes place in May, leaving a solid six months for the nominee to raise money and joust with former Sen. Bob Kerrey, the presumptive Democratic nominee.

Democrats have fewer competitive primaries in battleground states. Connecticut, New Mexico and Hawaii top the list, with the latter two more competitive than the Nutmeg State’s open seat. Hawaii and Connecticut also are August primaries.

The party’s ability to clear the field for its desired candidate in several states and its relatively large number of incumbents has kept its primaries to a minimum. But Republicans believe their chances have improved since the start of the cycle based on Democrats’ consensus candidates in a handful of key states.

That includes Reps. Shelley Berkley (Nev.) and Tammy Baldwin (Wis.) and Harvard University professor Elizabeth Warren (Mass.).

“In Nevada and Wisconsin, they’ve nominated the most liberal person they could get, as well as, obviously, in Massachusetts,” said Rob Jesmer, executive director of the National Republican Senatorial Committee. “So consequently it is going to be very detrimental to them in the general trying to get independent votes.”

Last cycle featured a string of surprising GOP primary results in Alaska, Nevada, Colorado and Delaware, leading to losses in the latter three states, which at one time looked like promising pickup opportunities. Democrats hope for a rerun, but Republicans say that’s just not in the cards this time.

“We have very good candidates running, and there really isn’t a case where I can think of where, if someone gets nominated, the seat’s off the table,” Jesmer said. “The flip side is true — we’re competitive in Wisconsin because they’re going to nominate the most liberal person [who] comes from the worst part of the state for a Democrat.”

Several Republicans are running in that swing state, including former Gov. Tommy Thompson, former Rep. Mark Neumann, former state Speaker Jeff Fitzgerald and a new entrant, venture capitalist Eric Hovde. In a presidential battleground, the question is whether the more moderate Thompson makes the race more competitive in November than the others.

Democrats argue that the field of Republicans in Missouri is so weak that Sen. Claire McCaskill, one of the most vulnerable incumbents in the country, will have an easier re-election. While none of the Republicans have turned in earth-shattering fundraising quarters, money likely won’t be an issue for businessman John Brunner, former state Treasurer Sarah Steelman, Rep. Todd Akin or whoever else might emerge from the
Aug. 7 primary.

“As someone who has a little knowledge in what we’re going to spend here,” Jesmer said, “I can assure people nobody is going to be outspent when they’re the nominee in Missouri.”

While Republicans were lining up to run in several states this cycle, including Florida and Michigan, some competitive primaries never materialized. Former Sen. George Allen is still expected to easily win the nomination in Virginia, the field quickly cleared in Montana for Rep. Denny Rehberg, and Ohio Treasurer Josh Mandel easily won the recent primary there. All, especially Virginia and Montana, are top pickup opportunities for the GOP.

Despite the disparity in the number of seats they are defending, Democrats also believe there is still time to expand the playing field. Party operatives see offensive opportunities to be had as a result of primary challenges for Republican seats.

Those include Indiana, where state Treasurer Richard Mourdock is challenging Sen. Dick Lugar, and Arizona, where Rep. Jeff Flake is the frontrunner to replace retiring Sen. Jon Kyl but is facing businessman Wil Cardon in the primary.

“In Arizona, which is probably the biggest race that is not quite on people’s radar yet, a self-funder is pledging to spend millions attacking Flake all the way until the end of August,” Cecil said.

The Indiana primary is different in a couple of ways. Mourdock has a better chance of winning than Cardon, but the state is less competitive for Democrats than Arizona, which President Barack Obama’s campaign is targeting.

Democrats have sought to label Mourdock a “tea party candidate” based on the fact that he is challenging Lugar from the right and is backed by conservative outside groups such as FreedomWorks and the Club for Growth. Mourdock’s main argument is that Lugar has simply been in office too long.

Democrats say Rep. Joe Donnelly would start out as the favorite against Mourdock. Regardless of whether that’s true, the race would certainly be more competitive and national Republicans would be more likely to have to spend there if Lugar is defeated.

The DSCC is backing Rep. Mazie Hirono over former Rep. Ed Case in Hawaii, where either candidate will have a strong opponent in former Gov. Linda Lingle (R) in Obama’s home state. The DSCC thinks Rep. Martin Heinrich, the favorite in the New Mexico primary over state Auditor Hector Balderas, will be strong against former Rep. Heather Wilson (R), whose primary troubles failed to materialize.

Perhaps the biggest question mark on the Senate landscape is in Maine, where former Gov. Angus King’s decision to run as an Independent — and to not make it clear which party he would caucus with in the Senate — ultimately resulted in a lackluster Democratic field and mostly silence from national party leaders.

At the onset of primary season, it should be noted that at this time in 2010, the drama that would unfold was barely on the political radar.

“In 2010, the U.S. Senate races were at the top of the ticket and they were really a byproduct of what had happened earlier in the cycle,” GOP strategist Chris LaCivita said. “It’s an entirely different cycle.”