Will Rogers once famously joked, ''I am not a member of any organized political party -- I'm a Democrat.'' If he were alive today, Rogers would have to switch parties to make that claim. It's the Republicans who are in open disarray.

On Tuesday night, Christine O'Donnell, an obscure conservative activist backed by Sarah Palin and the Tea Party, won Delaware's Republican Senate primary over the popular, moderate former governor Mike Castle in one of the year's great upsets. It followed a similar Tea Party triumph in Alaska, where the unheralded Joe Miller shocked the establishment by knocking off the incumbent senator, and scion of the state's leading political dynasty, Lisa Murkowski. Sharron Angle in Nevada, Rand Paul in Kentucky, Ken Buck in Colorado, and Mike Lee in Utah are other Tea Party-backed candidates who defeated mainstream favorites in GOP primaries. There can no longer be any question that the Tea Party is a force to be reckoned with. The question now is, at what cost?

The anger and frustration by Tea Party supporters were originally seen as likely to help Republicans in November, since midterm elections often get decided by the intensity of the parties' grassroots. Democratic voters registered little enthusiasm, and Republicans appeared set to take over one or both houses of Congress. They'll still gain seats in November, but it's harder to see how they'll win the Senate. The Tea Party is now less a blessing than a curse for Republicans.

O'Donnell is a prime example. The Delaware Senate seat was considered a sure bet to switch from Democrat to Republican, so popular was Castle among the state's broader electorate. O'Donnell, a litigious anti-masturbation activist and repeat candidate who drew 4 percent of the vote when she challenged then-senator Joe Biden in 2008, registered mainly as a local oddball who claimed that political opponents were hiding in the bushes outside her townhouse.

Delaware's GOP chairman has called O'Donnell ''delusional'' and said she could not be elected dog catcher. Political forecasters agree. ''O'Donnell's win takes Delaware off the table for Republicans and will make it significantly harder for them to net the 10 seats they need to win the majority,'' said Jennifer Duffy of the nonpartisan Cook Political Report.
The damage may not be limited to states with Tea Party candidates. There is also the issue of how the Tea Party's sudden prominence affects the national political climate. Before Tuesday, its candidates tended to be assessed as individuals, not collectively. But O'Donnell's win means that the movement probably has reached critical mass. If independent voters start to ascribe the views of Tea Party candidates like O'Donnell to Republican candidates in general, it could turn an election that was shaping up to be a referendum on President Obama into something more complicated, and therefore less favorable to Republicans' chances. The triumph of this right-wing populism could also arouse previously disenchanted liberal voters.

This week's races demonstrated the power of Tea Party activism to a degree certain to register among alarmed Republican lawmakers, who will be even less inclined to cooperate with Democrats. But it has also begun to demarcate electoral limits for the GOP that are proving tighter than most people imagined. This worries party leaders, who have already signaled that they won't support O'Donnell's candidacy.

If Republicans have started resembling Democrats in their thirst for intraparty conflict and ideological strife, they would do well to note how Democrats eventually overcame these crippling habits to gain the large majorities they currently hold. In 2006 and 2008, they made a conscious attempt to embrace candidates closer to the center, like the pro-life Senator Bob Casey of Pennsylvania, another political scion broadly popular in his state. In other words, they did the opposite of what Republicans are doing to their party by purging electable moderates like Mike Castle.

Any observer of national politics over the last generation should marvel at this reversal. As O'Donnell said in her unlikely victory speech, ''No more politics as usual!''

Joshua Green writes a weekly column for the Boston Globe.

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