THE DEFEAT OF Indiana Sen. Richard Lugar in Tuesday’s Republican primary is disappointing not only for the loss of a valuable legislator. It is particularly discouraging because of the confrontational, partisan attitude of the victor of the Senate primary, Indiana Treasurer Richard Mourdock. Most observers of Washington do not look at the capital’s gridlock and prescribe more partisanship and less cooperation. That, unfortunately, seems to be Mr. Mourdock’s view — both during the primary campaign and in his post-election comments.

Mr. Mourdock, attacking Mr. Lugar for excessive consorting with the Democratic enemy, ran television ads that excoriated the senator for the apparent sin of working with President Obama. “The time for being collegial has passed. It’s time for confrontation,” he told the New York Times last month. Mr. Mourdock, who will face Democrat Joe Donnelly in the fall, was no less bellicose after the returns were in, arguing that he would be bipartisan only in the sense of accepting Democrats’ acceding to the position of conservative Republicans. “I have a mind-set that says bipartisanship ought to consist of Democrats coming to the Republican point of view,” Mr. Mourdock told “Fox & Friends” on Wednesday morning.

This is a recipe for continued stalemate at a time when progress is essential. Mr. Mourdock is certainly entitled to his beliefs in limited government and lower taxes. But his my-way-or-the-highway attitude is not conducive to solving the problems, which Mr. Mourdock correctly identifies, of rising debt and growing entitlement spending. Bipartisanship does not require abandonment of deeply felt principles. It does accept that accommodation and compromise are a necessary part of the political process.

As Mr. Lugar noted in a statement, Mr. Mourdock’s “embrace of an unrelenting partisan mind-set” is “not conducive to problem-solving and governance.” Mr. Mourdock, he predicted, “will find that unless he modifies his approach, he will achieve little as a legislator. Worse, he will help delay solutions that are totally beyond the capacity of partisan majorities to achieve. The most consequential of these is stabilizing and reversing the federal debt in an era when millions of baby boomers are retiring. There is little likelihood that either party will be able to impose their favored budget solutions on the other without some degree of compromise.”

Mr. Lugar’s loss may have stemmed from some problems of his own making, such as his failure to maintain a residence in the state. At 80, he had served in the Senate for six terms. But the attacks from Mr. Mourdock and the outside conservative groups that poured millions of dollars into the primary contest involved issues and votes that showed the best of Mr. Lugar: his willingness to work across the aisle, including with President Obama; his vote for the Troubled Assets Relief Program at a critical moment in the economic crisis; his support of Supreme Court nominees Sonia Sotomayor and Elena Kagan. Mr. Lugar’s work on foreign policy, particularly nuclear disarmament, represented the essence of effective bipartisanship. He leaves a proud legacy in the Senate — and an important warning to the man who would succeed him.