Senate Republicans believe that the defeat of Minority Leader Tom Daschle (D-S.D.) on Nov. 2 sends a chilling message to Democrats who must stand for re-election in 2006 in “red states.”

Republicans not only point to Daschle’s loss at the hands of former Rep. John Thune (R) but also their pickup of five open seats in the South. All six of those states were carried by President Bush.

“One of the themes no matter whether one was in Cajun Country in Louisiana or in Florida or in the Black Hills of South Dakota was that this obstruction has to stop,” said outgoing National Republican Senatorial Committee Chairman George Allen (Va.). “There are consequences beyond the won-and-lost column in every election.”

The theory of heightened Democratic vulnerability will be tested in less than two years, as five Democratic Senators — Ben Nelson (Neb.), Bill Nelson (Fla.), Kent Conrad (N.D.), Robert Byrd (W.Va.) and Jeff Bingaman (N.M.) — stand for re-election in states that Bush carried on Nov. 2.

(The White House approached Ben Nelson about becoming Agriculture secretary recently, but he declined. If he had accepted, Republican Gov. Mike Johanns would have appointed a GOP Senator to fill out the term.)

Thune successfully turned the election into a referendum on Daschle’s role as leader of his party, which, he alleged, centered on blocking Bush’s agenda.

The lesson learned, Thune said, is that “if you are part of this pattern of obstruction and get yourself crosswise with the interests and values of your constituents, it is going to cost you politically.”

Even Democrats acknowledge that the defeat of the influential 18-year incumbent was a stunning blow and a cause for reflection.

But they add that Daschle’s position as leader of his party seeking re-election in a presidential year made for a unique set of circumstances that are unlikely to be repeated.

“You can overanalyze South Dakota, because in this case you had somebody who was a national leader,” said Democratic media consultant Karl Struble, who handled the television strategy for Daschle. For someone like Daschle, it is “easier to ascribe the national party agenda” to them.

Struble added, however, that “there is no doubt that being a Democrat in red states make you an endangered species.”

Democratic pollster Fred Yang added that “Democrats should all take pause [to consider] what happened to our candidates in the red states.”

To be sure, the GOP faces the opposite problem in a few contests. Republican Sens. Rick Santorum (Pa.), Lincoln Chafee (R.I.) and Olympia Snowe (Maine) must run for new terms in states won by Massachusetts Sen. John Kerry (D) in 2004.

The pivotal — but unknown — factor in each of these contests will be whether the party campaign committees can successfully convince top-tier candidates to challenge the incumbents.

While Senate Republicans’ four-seat pickup earlier this month was impressive, it could have been far broader had they not failed to cajole first-rate recruits into such red-state races as North Dakota, Nevada and Arkansas.

In South Dakota, for instance, strategists on both sides agree that if the GOP had put up any candidate other than Thune, Daschle would likely have been re-elected easily — as he had been in 1992 and 1998. Thune was well-known statewide from serving as the at-large House Member from 1996 to 2002 and from losing a high-profile race against Sen. Tim Johnson (D) last cycle by 524 votes.

South Dakota “was the perfect storm for us,” said Yang. “Bush won by a lot, there was a very polarized electorate and Thune was a great Republican candidate.”

However, finding a top-tier candidate to run against a long-serving and generally popular incumbent is never easy — especially in such small-population states as Nebraska, North Dakota and New Mexico.

In most small states, everyone in the political elite knows one another, and many ordinary citizens know members of the political elite. As a result, there is strong peer pressure against taking on a powerful figure, even if he or she belongs to a different party: If the challenger loses, he or she risks losing not just time and money but also friendships and working relationships.

A case in point is Delaware Rep. Mike Castle (R). In 2000, Castle considered taking on then-Sen. Bill Roth in the GOP primary. Roth, a long-tenured Finance Committee chairman, was slowed by age and looked potentially vulnerable, but Castle passed on the race, deferring to Roth and his legendary status inside the state. In a break from tradition, Tom Carper, the outgoing Democratic governor, challenged Roth instead and beat him.

At this point, only Nebraska’s Ben Nelson appears to have a top-tier opponent.

Johanns has repeatedly expressed an interest in running and has been encouraged to do so by Nebraska Sen. Chuck Hagel (R), who has made no secret of his distaste for Nelson. Johanns will be term-limited out of office in 2006.

The strength of a Johanns candidacy, coupled with Bush’s 35-point margin in Nebraska earlier this month, makes for tough sledding for Nelson, said the Senator’s communications director, David DiMartino.

“We are realistic that we have an uphill fight in a conservative state, but Ben Nelson’s record reflects Nebraska values and we will be proud to defend it,” DiMartino said.

On traditional wedge issues, Nelson is among the most conservative in his Caucus.

He is anti-abortion, pro-gun and voted both to end Democratic filibusters of federal judges proposed by Bush and in support of a ban on same-sex marriage.

“Senator Daschle’s challenge was he was forced to defend the positions of every member of the Caucus,” DiMartino said.

“Ben Nelson is no Tom Daschle,” agreed Struble, who aided the former Nebraska governor during his successful 2000 open-seat Senate race. “You can’t mistake them on wedge issues or tax policy.”

Outside of Nebraska, it’s less clear who’s going to run against the red-state Democrats.

Given Bush’s convincing 5-point victory in Florida and the Senate win by former Housing and Urban Development Secretary Mel Martinez, Florida Republicans are sure to focus their firepower on Bill Nelson.

Nelson won an open-seat contest with 51 percent of the vote in 2000, beating then-Rep. Bill McCollum (R), who was widely seen as too conservative for the general electorate. McCollum lost in a primary to Martinez in 2004.

Rep. Katherine Harris (R), the former Florida secretary of state, appears to be the leading candidate to take on Nelson, though a number of other candidates — including Lt. Gov. Toni Jennings, Attorney General Charlie Crist, Chief Financial Officer Tom Gallagher and former House Speaker Daniel Webster — are also mentioned.

In all likelihood, several names on that list will opt for the open-seat gubernatorial race instead.

Republican hopes in North Dakota appear entirely contingent on Gov. John Hoeven (R), who was comfortably re-elected to a second term this month.

Hoeven has given no indication of an interest in challenging Conrad, though — a situation that’s reminiscent of the failed GOP attempts to recruit former North Dakota Gov. Ed Schafer (R) to challenge Sen. Byron Dorgan (D) in 2004.

Schafer never seriously contemplated a bid; Republicans instead nominated little-known attorney Mike Liffrig, who lost to Dorgan by 36 points even as Bush was trouncing Kerry in the state.

Republicans seem even less likely to field serious challengers to Byrd and Bingaman, assuming they run for re-election in 2006.

Despite West Virginia’s increasing Republican tilt in federal races, Byrd is such a legend in the state and in the Senate that few would dare cross him. Bingaman doesn’t have the stature of Byrd, but the GOP in New Mexico is hobbled by a relatively weak bench, a reality that may give Bingaman a free pass.

The results of the 2006 contests in red states could be a make-or-break moment for the Senate Democratic Caucus as they look ahead to 2008 and beyond in hopes of returning to the majority they lost in 2002.

Five more Democratic Senators — Mark Pryor (Ark.), Tom Harkin (Iowa), Mary Landrieu (La.), Max Baucus (Mont.) and Tim Johnson (S.D.) — who sit in states Bush carried will be up in 2008.

Four Republicans will be up in Kerry states: Sens. Susan Collins (Maine), Norm Coleman (Minn.), John Sununu (N.H.) and Gordon Smith (Ore.).