If there were ever a senator a party would want to show the door, it's Arlen Specter. Personally disagreeable, philosophically unmoored and fundamentally self-interested, he represents the worst of the U.S. Senate.

So the collective cry of good riddance on the right that greeted his departure from the GOP is understandable. Specter joined the Republican Party in the 1960s for opportunistic reasons, and he left it last week for opportunistic reasons – a primary challenge from the talented conservative Pat Toomey that he probably wouldn't have been able to overcome.

A better politician wouldn't have so lost the affection and loyalty of his own party – and Specter didn't stake his career on any great matter of principle, as Joe Lieberman did on the Iraq war. On the issues, Specter was reliably unreliable. He dissented from his former party not just on social issues, but on everything else.

The danger to the GOP is making good riddance an ongoing rallying cry. South Carolina Republican Sen. Jim DeMint pronounced on Specter's departure, "I would rather have 30 Republicans in the Senate who really believe in principles of limited government, free markets, free people, than to have 60 that don't have a set of beliefs."

This is a bracing statement of suicidal purity. If Republicans had just 30 senators, it wouldn't matter if they scored 100 percent on the Ayn Rand score card – they'd be an irrelevant rump watching the Democrats pass something like the New Deal and Great Society rolled into one.

Besides, Republicans already have 40 senators, almost all of whom are quite conservative. All but three voted against President Barack Obama's stimulus bill, and they all opposed his budget. It's the other 20 they'd need to get to 60 who would be more heterodox. America is just too big and diverse a country for any party to forge an ideologically monochromatic majority. In their desperation a few years ago, Democrats acknowledged this reality and began recruiting pro-life and pro-gun Democrats around the country who would make Nancy Pelosi's constituents blanch.

If Republicans are going to come back in 2010, they'll need moderates like Rep. Mark Kirk in Illinois, Rep. Mike Castle in Delaware and former Rep. Rob Simmons in Connecticut to win Senate races. All of them would be to the left of DeMint but to the right of the Democratic alternatives in their blue states.

A "big tent" party needn't abandon its core convictions. Too often the phrase is a euphemism for becoming more socially liberal. Republicans who have thrived in the Northeast like New York Rep. Pete King and New Jersey Rep. Chris Smith show it's possible to be pro-life and bring a moderate, working-class sensibility to economic issues.

The simplistic media meme about Specter's defection is that he was pushed out by the Republican Party's harsh lurch to the right during the George W. Bush years. Never mind that many of Bush's central domestic initiatives weren't particularly conservative – from No Child Left Behind to the prescription-drug benefit – and the party isn't any more socially conservative than it was when it enjoyed successes in the 1980s and 1990s.

The GOP has been brought low by the Bush administration's perceived incompetence, an unpopular war and its lack of new policy ideas, especially when it comes to addressing middle-class economic anxieties. The latter is the key challenge going forward and one to which moderates like Specter have contributed nothing. The party awaits its next Jack Kemp, who died over the weekend.

The former quarterback revived his party during its travails in the 1970s. He wasn't afraid to challenge a stale Republican policy consensus (in favor of deficit reduction) with new ideas (supply-side tax cuts) designed to appeal to a key electoral constituency (hard-pressed middle- and working-class voters).

Kemp never sounded angry or embattled. His unabashed conservatism was tonally open and fresh, about growth both economically and electorally.

Specter's bolt to the Democrats need signify nothing other than the latest twist in his endlessly flexible career – provided Republicans find another Jack Kemp to, in Lincoln's words, think and act anew.