Tuesday in Delaware was a bad day not only for Republicans but for conservatives. Tea partier Christine O’Donnell scored a stunning victory over establishment Republican Mike Castle. Stunning but pyrrhic. The very people who have most alerted the country to the perils of President Obama’s social democratic agenda may have just made it impossible for Republicans to retake the Senate and definitively stop that agenda.

Bill Buckley – no Mike Castle he – had a rule: Support the most conservative candidate who is electable.

A timeless rule of sober politics, and particularly timely now. This is no ordinary time. And this is no ordinary Democratic administration. It is highly ideological and ambitious. It is determined to use whatever historical window it is granted to change the country structurally, irreversibly. It has already done so with Obamacare and has equally lofty ambitions for energy, education, immigration, taxation, industrial policy and the composition of the Supreme Court.

That’s what makes the 11th-hour endorsements of O’Donnell by Sen. Jim DeMint, R-S.C., and Sarah Palin so reckless and irresponsible.

Of course Mike Castle is a liberal Republican. What do you expect from Delaware? A DeMint? Castle voted against Obamacare and the stimulus. Yes, he voted for cap-and-trade. That’s batting .667. You’d rather have a Democrat who bats .000 and who might give the Democrats the 50th vote to control the Senate?

Castle wasn’t only electable. He was unbeatable. Why do you think Beau Biden, long groomed to inherit his father’s seat, flinched from running? Because Castle, who had already won statewide races a dozen times, scared him off. Democrats had already given up on the race.

O’Donnell, a lifelong activist who has twice lost statewide races, is very problematic. It is not that the Republican establishment denigrates her chances – virtually every nonpartisan electoral analyst from Charlie Cook to Larry Sabato to Stuart Rothenberg has her losing in November.

Nor is opposition to O’Donnell’s candidacy a sign of hostility or disrespect to the tea party. Many of those who wanted to see Castle nominated in Delaware have from the beginning defended the tea party movement from the mainstream media’s scurrilous portrayal of it as a racist rabble of resentful lumpenproletarians. Indeed, it is among the most vigorous and salutary grass-roots movements of our time, dedicated to a genuine constitutionalism from which the country has strayed far.

And its complaint that it is often taken for granted by the Republican establishment (interestingly parallel to the often-heard African-American community’s complaint against the Democratic Party) is not to be dismissed. Tea partiers should not, as many of them fear, simply be used by the Republican Party as a source of electoral energy while their own candidates are ignored and dismissed. But the question is: Which of their candidates?

Marco Rubio in Florida is strong, serious, dynamic. He has a great future as a Republican leader. Joe Miller, who upset the Murkowski dynasty in Alaska, is a man of remarkable achievement: West Point graduate, decorated veteran, judge. Both will win.

Moreover, geography matters. Rand Paul may not be the best candidate in the world – it is not a very good idea to start your general election campaign by expressing reservations about the Civil Rights Act – but he is running in Kentucky. He will almost certainly win.

Delaware is not Kentucky. If Republicans want to be a national party, they cannot write off the Northeast, whose Republicanism is of a distinctly moderate variety. Scott Brown broke Republican ranks to vote for Obama’s financial reform. Are conservatives going to now run him out of the Senate? Wasn’t it just eight months ago that his victory in Massachusetts was hailed as a turning point in the campaign to stop the Obama agenda?

You don’t stop that agenda by nominating an O’Donnell in Delaware and turning a Senate seat from safe Republican to safe Democratic.

If DeMint and Palin want to show that helping O’Donnell over the top – she won late and by six points – wasn’t a capricious spreading of fairy dust, perhaps they should go to Delaware now and get her elected to the Senate.

You made it possible. Now make it happen. I would be happy to be proved wrong about O’Donnell’s electability – I want Republicans to win that 51st seat. Stay in Delaware and show us you were right. The beaches are said to be lovely in the fall.