PROVIDENCE -- Just two Republicans have represented Rhode Island in the U.S. Senate since the second Franklin D. Roosevelt administration in the 1930s. Both of their surnames have been Chafee -- John H. Chafee, who served from 1976 until he died in office in 1999, and his son Sen. Lincoln D. Chafee, who was appointed to the seat after his father's death and won it outright by a huge margin in 2000.

Now Chafee, arguably the most liberal Republican in the Senate, is locked in a tough and increasingly caustic joust for renomination against Cranston Mayor Stephen Laffey, a feisty conservative backed by the Washington, D.C.-based Club for Growth, an antitax group.

On Tuesday, voters in the state GOP Senate primary will decide Chafee's future in an election that has drawn national attention because it may be crucial in deciding whether Republicans retain control of the Senate.

Both camps describe the election as close; there are no reliable independent polls. If the past is prologue, the winner will be the candidate who is able to motivate his supporters to vote. Historically, Republican primaries in Democratic Rhode Island have drawn but tiny slices of the state's 660,000 voters; the record modern GOP turnout is about 45,000, from a 1994 race for governor.

Laffey, 44, has been Cranston mayor for two terms. He is known for his aggressive administrative style, especially in dealings with public employee unions, and his energetic, door-to-door campaigns.

Chafee, 53, is a scion of one of New England's storied Yankee families. He is a former Warwick city councilman and mayor. He was the lone Republican senator to vote against President Bush's prosecution of the Iraq War.

The Laffey-Chafee showdown will determine -- at least in the short run -- whether the Rhode Island GOP will turn right, or remain in the moderate New England camp.

There is also a Democratic Senate primary Tuesday. It has not drawn nearly as much attention as the GOP contest, because it is not expected to have national implications or provide a surprise.

The Democratic favorite is the party's endorsed candidate, 50-year-old Sheldon Whitehouse of Providence, a Yale University and University of Virginia Law School graduate, who boasts a long government résumé. Whitehouse was an assistant attorney general; legal counsel to former Gov. Bruce Sundlun during the state credit union crisis of the early 1990s; director of the state Department of Business Regulation; U.S. Attorney for Rhode Island, and state attorney general.

Whitehouse lost a 2002 bid for governor but has rebounded well, harvesting more than $4 million in campaign money and generating support via a series of more than 30 community dinners and cookouts thorughout the state. He bills himself a "mainstream Democrat" who says he would be a senator in the mold of the Democrats who represented the state for most of the 20th century -- Claiborne Pell, Theodore Francis Green, John Pastore and incumbent U.S. Sen. Jack Reed.

Polls have shown Whitehouse with a big general election lead over Laffey but facing a tight race if Chafee is the GOP nominee.

Whitehouse has faced a spirited, albeit underfinanced, challenge, from 46-year-old Carl Sheeler of West Greenwich, a U.S. Marine veteran and business owner who has campaigned as a sharp critic of President Bush and U.S. policy in Iraq. Sheeler has never held public office before and has not had sufficient campaign money to run the high-profile television commercials that have been a staple of Whitehouse's effort.

Christopher Young of Providence, who is also running in the primary for the Democratic Providence mayoral nomination, is the third candidate in the Democratic Senate race.

As the weeks winnow to the Nov. 7 general election, Rhode Island's Senate campaign will remain in the spotlight, says Jennifer Duffy, who follows Senate elections for the Washington, D.C-based Cook Political Report.

Democrats need to win six Republican seats to become the Senate's majority party.

"In the math problem that is the Democratic road to a Senate majority, it is not an exaggeration to say that there is no way the Democrats can regain Senate control without winning in Rhode Island," says Duffy.

smackay@projo.com / (401) 277-7321