RAPID CITY, S.D. ? Shattering precedent, Senate Majority Leader Bill Frist campaigned for the defeat of his Democratic counterpart on Saturday, depicting Sen. Tom Daschle as a polarizing figure at home and an obstructionist thwarting President Bush’s agenda in Congress.

Former GOP Rep. John Thune “would be a perfect United States senator to serve the people of South Dakota,” said Frist, standing next to Daschle’s rival. Blending local concerns with national issues, he pledged to work with Thune to find a new mission for a nearby Air Force base threatened with closure.

Cross-country campaigning is common for prominent and ambitious politicians such as Frist, a Tennessean whose current tour will take him to six states in hopes of enlarging the GOP majority this fall.

But when he arrived in South Dakota — the leader of one party trying to deny the leader of the other a new term — he made this trip unlike any other.

“We haven’t found anything in our files” that is comparable, said Don Ritchie of the Senate historian’s office. The only thing close occurred a century ago, he said, when the leader of one wing of the Republican party campaigned to defeat the leader of the other.

One Democrat took exception to Frist’s politicking.

“What has become of civility? … It used to be unheard of for Senate leaders to seek an active role against each other in campaigns,” said Sen. Robert Byrd of West Virginia, who has served for more than 45 years and led his party for a dozen of them. “That time has gone,” he lamented.

“It may be rare, but these are rare times,” Frist countered at a news conference.

In a reference to polls showing a competitive race between Daschle and Thune, the senator added, “It is rare to have the leader of a party … not be very strongly supported by the people in their home state.”

Anticipating continued GOP control of the Senate after this year’s elections, as well as a second term for Bush, Frist said South Dakota voters could choose a candidate who believes in the president and his leadership, or stick with a senator who has been “obstructing in the United States Senate,” everything from energy legislation to welfare.

Daschle is seeking a fourth terms in his Republican state. Thune served six years in the House before giving up his seat for an unsuccessful Senate campaign in 2002. Bush is expected to win the state against John Kerry, the presumptive Democratic nominee.