When Indiana Republicans go to the polls on Tuesday, they will do more than choose a candidate for the Senate. They will choose between party and country.

That’s a stark assessment but true. On one side is a man who has made it his life’s work to build a cross-aisle consensus for keeping nuclear weapons out of the hands of terrorists and rogue states. On the other side is a man who mocks his opponent for such efforts and who talks more about fighting Democrats than fighting America’s enemies.

For years Dick Lugar has been the leading Senate Republican on foreign policy, shaping post-Cold War strategy, securing sanctions to end South African apartheid and bringing democracy to the Philippines, among other things. His signature achievement, drafted with Democrat Sam Nunn, was the 1992 Nunn-Lugar Act, which has disarmed thousands of Soviet nuclear warheads once aimed at the United States.

Enter Richard Mourdock, a tea party hothead attempting to defeat Lugar in the GOP primary. A cornerstone of his effort to oust Lugar is the six-term senator’s bad habit of bipartisanship — never mind that Lugar’s bipartisanship was in the service of protecting millions of Americans from nuclear, chemical and biological terrorism.

In one typical ad, Mourdock’s campaign plays a clip of Barack Obama saying, “I’ve worked with Republican Senator Dick Lugar to pass a law.” And then a clip of Obama saying, “What I did was reach out to Senator Dick Lugar.”

Deviously, Mourdock’s ad cuts off the clip before the viewer can learn what the law was about. In the first instance, Obama said: “I’ve worked with Republican Senator Dick Lugar to pass a law that will secure and destroy some of the world’s deadliest, unguarded weapons.” In the second instance, Obama said: “What I did was reach out to Senator Dick Lugar, a Republican, to help lock down loose nuclear weapons.”

Those details omitted, this ad — and variations of it posted by the Mourdock campaign over the past year — goes on to show a cartoon Lugar dancing and giggling with a cartoon Obama, pink valentines between them and a rainbow (symbol of the gay pride movement) above them. “Some say he has even earned the title of President Obama’s favorite Republican senator,” Mourdock says in the ad. “My friends, I can tell you that is a title I will never hold!”

Indeed, Mourdock, Indiana’s state treasurer, boasts about his refusal to work with Democrats. “The time for being collegial is past,” he told the New York Times recently. “It’s time for confrontation.”

There is a great deal to dislike in Mourdock’s message, but the most egregious part is his underlying contention that Lugar should be punished for cooperating with the other party — even though such cooperation protects the country against unimaginable destruction. That’s not just wrong; it’s unpatriotic.

The legislation Obama and Lugar drafted in 2006 expanded the original Nunn-Lugar Act to cover conventional weapons stockpiles. It was wrapped into a House bill and proved so uncontroversial that it passed by a voice vote there and by unanimous consent in the Senate.

The Mourdock campaign attributes Mourdock’s claim that Lugar is “President Obama’s favorite Republican” to MSNBC, apparently referring to an online report before Obama won the presidency that was headlined “Barack Obama’s favorite Republican?” The report noted that Lugar “is a loyal Republican and McCain supporter.”

The deeper implication — that Lugar isn’t a conservative — is at odds with Lugar’s 77 percent lifetime rating by the American Conservative Union (75 percent last year). That hardly makes him a moderate (the ratings of Maine’s Republican senators hover around 50 percent), but among the sans-culottes in the modern Republican Party, anything less than purity merits purging.

Mourdock hits Lugar for voting in favor of Obama’s two Supreme Court nominees; never mind that Lugar also voted for all five conservative justices. Mourdock condemns Lugar for supporting the START Treaty, the assault weapons ban and theDream Act. Lugar also voted against Obama’s stimulus, health-care plan and financial reforms. “His bipartisanship, his willingness to push the Obama agenda, has caused him to be labeled President Obama’s favorite Republican senator,” Mourdock falsely alleges in one ad.

As Mourdock piles up support from the likes of Sarah Palin, Michele Bachmann and Grover Norquist, Lugar is still clinging to the notion that substance matters; this week, his office issued a news release titled “Lugar Announces Elimination of Weapons of Mass Destruction through Nunn-Lugar.”

Some Democrats hope that Mourdock beats Lugar because it would increase the likelihood that Democrat Joe Donnelly will win the seat in November. But that’s not why Hoosier Republicans should reject Mourdock. They should reject him because they still believe that national security trumps partisanship.

danamilbank@washpost.com