Senate Majority Leader Bill Frist broke protocol, and bruised some feelings, by campaigning here Saturday for a goal that has not been achieved in half a century: defeating a Democratic or Republican Senate leader seeking reelection.

Frist (R-Tenn.), who is heading a massive fundraising effort to deny Minority Leader Thomas A. Daschle (D-S.D.) a fourth term, put his feet where his money is Saturday, traversing the state twice in 10 hours to stump for Republican John Thune. Historians say they know of no previous majority or minority leader who traveled to his counterpart's state to campaign for his ouster.

But Senate tradition and decorum are melting this year beneath the Senate's bitter partisanship, the GOP's zeal to keep its razor-thin majority, and the party's belief that it can knock off Daschle in a state that voted overwhelmingly for President Bush in 2000.

Republicans, who hold 51 of the Senate's 100 seats, are running hard to replace five retiring Democrats from the South, even as they struggle to hold GOP seats in Colorado, Oklahoma, Illinois and Alaska. Although every seat is precious, nothing would delight Republicans more than gaining a South Dakota seat and ousting the man they blame for thwarting Bush's judicial nominees and other GOP goals.

Thune, a former House member, came within 524 votes of beating Sen. Tim Johnson (D-S.D.) two years ago. Analysts viewed that costly race as a surrogate struggle between Bush, who recruited Thune to run, and Daschle, Johnson's mentor. This time, Daschle is on the ballot, and the GOP establishment -- from the White House down -- is making his defeat a priority second only to Bush's reelection.

Frist on Saturday followed the president, the first lady, the House speaker and other top Republicans who have paraded to South Dakota to argue that Thune is better suited to represent a state where Al Gore got only 38 percent of the 2000 presidential vote and GOP state legislators outnumber Democrats 75 to 30. Frist kept the visit low-key, generally referring directly to Daschle only when prodded by reporters. He said his campaign visit was justified by the Senate's narrow division and by the Democrats' refusal to embrace some of Bush's judicial nominees, tax cuts and energy policies.

"These are unusual times," Frist said after touring the Dell Rapids Community Hospital, north of Sioux Falls. "One vote may make the difference as to whether or not we get comprehensive energy legislation." Pointing to Thune, he said, "I believe in supporting people who you believe in now, who you've believed in in the past, and who you will believe in in the future."

Thune criticized Daschle for failing to stop a Democratic filibuster of a Republican-crafted energy bill that Daschle endorsed. "That was a complete and utter failure of leadership," Thune said.

While Frist soft-pedaled his criticisms of Daschle on Saturday, he is robustly raising money for Thune. His recent mass-mail fundraising letter says, "If you only can make one more contribution to one of our Republican Senate candidates this election cycle, you should make that gift to John Thune!"

Democrats are mobilizing to fight back. "No one needs to be reminded that Tom is the number one target of the national Republicans in this cycle," said Joel Johnson, a lobbyist and former aide to Daschle and President Bill Clinton. "The Republicans are pulling out all the stops because they realize that's the only way they have a shot at winning."

Thune has reasons to feel hopeful, some political activists here say. He is running a sharper, better-staffed campaign than he did two years ago, they say. A recent poll by the Sioux Falls Argus Leader and KELO-TV showed Daschle with a statistically meaningless lead over Thune, 49 percent to 47 percent. That is worrisome for Daschle, who has aired television commercials frequently since last summer whereas Thune has aired none.

Remarkably, the poll found only 4 percent of the state's voters are undecided. With each campaign planning to spend about $10 million on the race, they can devote a combined $1,495 to each undecided voter if the Nov. 2 turnout is similar to that of the Johnson-Thune contest.

Even before Frist jetted from Rapid City to Sioux Falls and back, he was aiding 20 Washington-based lobbying groups that have formed Team Thune to raise at least $500,000 for the GOP challenger. Last Tuesday, the group hosted a fundraiser at the Willard Hotel that collected $149,000. White House Chief of Staff Andrew H. Card Jr. was the featured speaker.

Team Thune has scheduled other fundraising events and plans to mobilize allies in South Dakota. Team members include the Associated Builders and Contractors, the International Food Service Distributors Association, the National Electrical Manufacturers Association, the National Restaurant Association, the National Roofing Contractors Association, Printing Industries of America and the Property Casualty Insurers Association of America.

Frist's visit here has rankled some veteran senators, such as Robert C. Byrd (D-W.Va.). In a Senate speech last month, he said: "It used to be unheard of for Senate leaders to seek an active role against each other in campaigns. . . . Who cares about honor when a Senate seat might be gained?"

Daschle, who was not in South Dakota on Saturday, has shown more restraint. Frist "has got to do what he's got to do, and I won't challenge his right to do it," he said in a recent interview. "He's welcome to South Dakota. I've asked him to address not only the politics of the year but the issues that many people in South Dakota are concerned about."

Daschle particularly urged Frist to help keep the administration from closing Ellsworth Air Force Base, near Rapid City. Frist toured the base Saturday morning, and suggested it might make a good home for unmanned aircraft being developed by the Pentagon. Daschle campaign spokesman Dan Pfeiffer said the best way for South Dakotans to keep the base open is to reelect the Democratic leader, who makes at least one appointment to the federal commission that recommends base closures.

Frist also attended a Thune fundraiser in Sioux Falls and spoke at Republican Lincoln Day dinners there and in Rapid City. While his visit caused a stir on Capitol Hill, it seemed to generate little passion in this state, already flooded with political ads for a June 1 special House election.

"We've had a conga line of political celebrities come through this state," Rapid City Journal political reporter Denise Ross said. "Senator Frist is just another one in the line."

Birnbaum reported from Washington.

Senate Majority Leader Bill Frist (R-Tenn.), left, appears Saturday in Box Elder, S.D., with GOP Senate candidate John Thune. Republicans have made defeat of Minority Leader Thomas A. Daschle a priority in the fall elections.