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Throwing tradition out the window, Majority Leader Bill Frist (search) took the unprecedented step last weekend of openly stumping to unseat his Democratic counterpart Tom Daschle (search) of South Dakota.

"What it indicates now is probably that things have become more partisan," said Senate Associate Historian Don Ritchie. Ritchie said leaders have traditionally laid off campaigning against one another because they would have been "trying not to burn too many bridges."

Partisanship is nothing new in Washington, D.C., but the Senate was designed to be somewhat insulated, a gentleman's club of sorts where collegiality and civility took priority over party politics. In the past, that cordiality has made it rare for sitting senators to campaign against one another, and according to chamber watchers, virtually unheard of for Senate leaders.

In recent decades, however, senators, but not party leaders, have started to campaign against one another.

"It used to be unheard of for a senator to campaign against a colleague by going to that colleague's state. That norm was eroded significantly over the last 20 years. For a leader to campaign against another leader is unprecedented — but that is in part because leaders rarely have tough races," said American Enterprise Institute (search) political expert Norm Ornstein.

Frist is doing his best to see Daschle go, traveling to South Dakota to stump for Republican John Thune (search). Frist said he is merely doing his job of trying to build and protect a slim Republican majority.

"One vote matters. Policy matters. Elections matter," Frist said while on the road last Saturday.

The historical rarity of Frist's visit, however, suggests another story. It has been more than 50 years since a majority or minority leader has lost re-election while head of his caucus.

Ritchie said without having done a systematic search, the closest similarity to current events occurred in 1900.

Although this was an era before formal minority and majority leaders, Ohio Sen. Marcus Hanna, a national Republican leader, was spurred to get on a train and campaign against the re-election of South Dakota Sen. Richard Pettigrew, a leading member of the Silver Republicans (search), a group that split from the main party in a dispute over making coins out of silver.

The Senate is structured differently from the House, in a way that the minority can block much of the legislation, particularly in a closely split session. Sixty votes are needed for cloture — to end debate — and an individual senator has the ability to filibuster, delaying legislation.

"It's different in the Senate than in the House because the two parties have to work closer together, and especially the two leaders have to work close together," Ritchie said.

But partisanship has grown as diversity of opinion within the parties has declined, Ritchie said, making cooperation across party lines more difficult. In the past, more moderates populated each party, allowing for more cross-party alliances.

"Until 20 years ago, the two parties were evenly divided internally with moderate wings," Ritchie said. "That's the reason why there were almost no party-line votes. Now the two parties are internally very coherent and consistent."

Senate Republicans are also inspired to fight for every seat because the body is so closely divided. Currently, the Senate is comprised of 51 Republicans, 48 Democrats and one Democratic-leaning independent. Earlier in the year, Republicans had felt confident they would be able to maintain and even expand the lead, but Democrats have effectively recruited a field of competitive candidates making the outcome of November's election less certain.

Daschle skated to re-election in 1998, with 62 percent of the vote, but Thune is a formidable challenger, having lost against South Dakota Sen. Tim Johnson in 2002 by just 524 votes. Daschle, who is running for his fourth term, is taking nothing for granted and has been blanketing South Dakota media with advertisements delivering his messages.

The race is close, according to local polling. A Sioux Falls Argus Leader/KELO-TV poll conducted May 10-12 of 800 likely voters showed a virtual dead heat, with Daschle at 49 percent and Thune at 47 percent.

Daschle appeared nonplussed by Frist's visit, betraying no bitterness in his public comments. "He's welcome, and I only hope he will make it a substantial as well as political trip," Daschle told reporters when word of the trip surfaced. He sent the Tennessee senator a letter last week, encouraging him to work on policy, not just politics on his visit to the Mount Rushmore State.

Frist said his trip has nothing to do with Daschle, but instead is about retaking the Senate. He also points out that he stumped for Thune last time around and it would be a bit disingenuous if he didn't aid him again.

During his visit, Frist told South Dakotans that casting their vote for Thune could preserve the Republican majority. Still, he acknowledged that campaigning against a sitting Senate leader is rare. "But these are rare times," he said.

But not everyone accepts that politics should be allowed to supercede longstanding Senate habits.

Looking back over his 45 years of Senate experience, Sen. Robert C. Byrd, D-W.Va., presciently remarked on the tactic during an April 28 Senate floor speech.

"It used to be unheard of for Senate leaders to seek an active role against each other in campaigns. That time has apparently gone. Has honor gone, too? Who cares about honor when a Senate seat might be gained?"