TALLAHASSEE, Fla. – The race for Florida’s open Senate seat could play a huge role in shaping the nation’s political landscape in the years to come: It could determine which party controls the Senate, and even influence the presidential race.

That is the easy part.

Determining who will emerge from the August 31 primary is an entirely different story. Seven Republicans are on the ballot in the race to succeed retiring Senator Graham, a Democrat, while three prominent Democrats are locked in their own close contest.

The race has drawn heavy interest from national parties as they jockey for control of the Senate, which has 51 Republicans, 48 Democrats, and one Democrat-leaning independent. Florida is one of eight states with open Senate seats this year.

“It’s right in there on the A-list,” said Senator Allen of Virginia, chairman of the Republican Senatorial Campaign Committee. “This is going to be a very, very close race and you have superimposed upon it the fact that this is going to be one of the battleground states in the presidential race as well.”

The Republican contest is a departure from the last time Florida had an open Senate seat. In 2000, the party made sure its nominee had an easy primary, throwing all of its support behind then-U.S. Rep. Bill McCollum only to have him lose to Democrat Bill Nelson.

No such GOP unity exists this year. Mr. McCollum is running again, but Washington seems to have a new favorite in Mel Martinez, who served as Housing and Urban Development Secretary under President Bush. Though Mr. Martinez is running a close second behind Mr. McCollum, his fund-raising prowess and ties to the White House could make him tough to beat.

“Anybody that’s mildly aware of current events has the feeling that President Bush wanted (Martinez) to run for the seat,” said Paul Bedinghaus, the Pinellas Country Republican chairman. “That gives him a certain entree to fund-raising people and it causes grass-roots voters to perk up and be more curious about him.”

Doug Gallagher, a self-financed millionaire who has never held elected office, is running a distant third in the GOP race but is rising in the polls.

On the Democratic side, a former Education Commissioner, Betty Castor, leads a field that includes Rep. Peter Deutsch and Miami-Dade County Mayor Alex Penelas.

Ms. Castor, a former state lawmaker, has been riding high in the primary for more than a year, helped by a centrist message and goodwill earned from years of experience in Tallahassee and as president of the University of South Florida.

But Mr. Deutsch has a hefty campaign account and an aggressive campaign style. Most recent polls have given Ms. Castor a double-digit lead over Mr. Deutsch, but about 30% remain undecided. Mr. Penelas’s campaign has struggled to gain traction.

Democrats hope they can hold onto the seat held by Mr. Graham, a political icon in Florida. He defeated an incumbent Republican to win office in 1986 after serving two terms as governor. He then twice won re-election by wide margins, and few doubt he would have easily defended his seat had he decided to seek a fourth term.