The battle for the U.S. Senate in 2008 will become much clearer shortly after Labor Day.

Three incumbents and one challenger will make decisions that will have a major impact on the election. Three are incumbents -- two Republicans and one Democrat -- who will shortly decide whether or not to seek a new term. The fourth is a Democratic challenger who could give a Republican incumbent a real run for his money.

Let’s look at each race:

New Mexico: Incumbent Republican Sen. Pete Domenici, age 75, faces a decision on seeking a new term. If he chooses to retire, it is possible that Gov. Bill Richardson would drop out of the presidential race and run for the Senate. If he declines, it is possible that Congressman Tom Udall (whose brother Marc is running for the Senate in Colorado) could enter the race. Congresswoman Heather Wilson is a possible Republican candidate which would open her seat to a strong Democrat.

New Hampshire: Incumbent Republican Sen. John Sununu could face a real fight if former Governor Jean Shaheen decides to challenge him. Shaheen is currently the Director of the Institute of Politics at the Kennedy School of Government at Harvard and has been consistently running 10 points ahead of Sununu in polling. This would be a re-match of the 2002 race. Shaheen, who likes her current position at Harvard, is being heavily courted by Senate Democrats. Even if Shaheen doesn’t run, Sununu could have a significant challenge.

South Dakota: Incumbent Democratic Sen. Tim Johnson has been convalescing from a stroke he suffered in December and has not yet returned to work. Should he not run, this would be a wide-open race with Congresswoman Stephanie Herseth Sandlin a potentially strong Democratic candidate and Republican Gov. Mike Rounds a potential GOP contender. Out of respect for Johnson, politicians of both parties have been very circumspect though a decision from Johnson is likely soon.

Virginia: Republican Senator John Warner, age 80, has not indicated whether he will seek an additional term. Should he not run, this also would be a free-for-all with former Democratic Governor Mark Warner a real possibility and Republican Congressman Tom Davis a strong candidate for his party.

Senator Warner is playing a key role in formulating future Iraq policy and might see that as his swan song.

The press has been focusing on the presidential race to the exclusion of virtually everything else in politics right now, but decisions by these four individuals will help shape the playing field for next year. Each of the three possible open seats would be highly competitive and the New Hampshire race would be the number one challenge race in the entire country if Shaheen runs.

Open seats in New Mexico and Virginia also could have an impact on the presidential contest. New Mexico clearly is a swing state and a strong showing by the Democratic Senate nominee could tip the balance in favor of the Democratic presidential nominee. Virginia is on the cusp of becoming a real opportunity state for Democrats in presidential years and a strong showing by a Democratic Senate candidate might tip Virginia into the Democratic column for president this year.

Maybe, at least for a while after Labor Day, the national press corps will turn away from the presidential race and pay attention to very important decisions which are about to be made in these four states.

It would be refreshing to read about something else for a while and these races actually matter. Democrats only hold a one-vote margin in the U.S. Senate and retirement decisions could play a big role in firming up the Democrats position.

For that reason, you can expect heavy Republican pressure to be applied to Warner and Domenici and that equally heavy pressure will be directed by Democrats on Shaheen. Everyone will leave Tim Johnson alone for a while longer, but eventually that race also will have to be clarified.

There is something other than presidential politics in 2008 and these four individuals will make a big difference when they finally declare.

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Martin Frost served in Congress from 1979 to 2005, representing a diverse district in the Dallas-Ft. Worth area. He served two terms as chairman of the House Democratic Caucus, the third-ranking leadership position for House Democrats, and two terms as chairman of the Democratic Congressional Campaign Committee. Frost serves as a regular contributor to FOX News Channel and is a partner at the law firm of Polsinelli, Shalton, Flanigan and Suelthaus. He holds a Bachelor of Journalism degree from the University of Missouri and a law degree from the Georgetown Law Center.