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UPDATE: As predicted, Lugar lost his primary challenge to Mourdock, 60 percent to 40 percent. Ad Policy

If Indiana Senator Dick Lugar loses his Republican primary race to Tea Party challenger Richard Mourdock tonight, as polls indicate is likely, his defeat will signal the end of moderate Republican internationalism in the US Senate and the GOP more broadly.

Lugar, a two time chairman of the Senate Foreign Relations Committee best known for his work on arms control and nonproliferation treaties, used to be one of the GOP’s leading figures on foreign policy. Now he’s an outlier.

The Senate Republican caucus was once filled with the likes of Dick Lugar—sensible realists such as Lincoln Chafee, Chuck Hagel, George Voinovich and Olympia Snowe. Now they’re all gone or going, casualties of a Republican party where diplomacy, multilateralism and bipartisanship are dirty words. (A Mourdock ad called Lugar “Obama’s favorite Republican.”)

The decline of Lugar’s brand of pragmatic internationalism on foreign affairs helps explain why neoconservative veterans of the Bush Administration are now the principal foreign policy advisers to Mitt Romney. As I wrote in the latest issue of The Nation:

Elder statesmen from the George H.W. Bush administration like [Colin] Powell and [Brent] Scowcroft are much closer to Obama than to Romney. “The foreign policy experts who represent old-school, small-c conservatism and internationalism have been pushed out of the party,” says Heather Hurlburt, executive director of the center-left National Security Network. “Who in the Republican Party still listens to Brent Scowcroft?” Lawrence Wilkerson, former chief of staff to Powell, says the likes of Powell and Scowcroft are “very worried about their ability to restore moderation and sobriety to the party’s foreign and domestic policies.”

Scowcroft, the former national security adviser to George H.W. Bush, elaborated in a recent interview with Fareed Zakaria. “I’ve been called a RINO, a Republican in Name Only,” he said. “I don’t think I’ve changed at all. I think the party has moved.”

In March, Hagel was asked, “Do you still consider yourself a Republican?” He responded, “I don’t know what the Republican Party is.”

On the contrary, I’m guessing that the likes of Hagel and Lugar know all too well what the GOP has become.

Ari Berman is the author of Herding Donkeys: The Fight to Rebuild the Democratic Party and Reshape American Politics, now out in paperback.