"Whether you agreed with him or not, whether he had the gavel or whether he didn't have the gavel, Dick Lugar had an approach to the Senate and to governing that was always the same. He was serious, he was thoughtful and he refused to allow this march to orthodoxy about ideology and partisan politics to get in the way of what he thought was the responsibility of a senator and indeed the need of the country to have people come together and find common ground."

Lugar was defeated by Indiana state Treasurer Richard Mourdock, a Tea Party favorite, in the state's Republican primary on Tuesday.

Kerry praised Lugar's "expertise" on certain foreign policy issues.

"Mr. President, I am confident that Dick Lugar's record on our committee will be one of those that's remembered for a long time," Kerry said. "Sadly, last night, it is remembered in the context of Sen. Fulbright who also came to lose a primary in the end and paid a high price for his concern about global affairs and his involvement in those issues. But I think that he's also remembered today significantly for the enormous legacy he built about foreign policy. Dick Lugar does that and I think he's made that clear — I mean there's no doubt in the mind of anybody on our side of the aisle that Dick Lugar is a conservative. And his votes through the years have shown that. He's a proud Republican."

Durbin came onto the flight immediately after Kerry and said he strongly agreed with Kerry on their Republican colleague. Durbin praised "the rebirth of Indianapolis" as due in part to Lugar earlier in his political career.

"I know that Dick Lugar in the remaining months is going to be an extraordinary servant of people in Indiana and the nation as he has been throughout his career and I look forward to seeing him here back on the Senate floor working, as he will, for the remainder of his term," Durbin said. "But it is a loss. It's a loss to the Senate that he's leaving and it's a sad day on both sides of the aisle that Dick Lugar won't be part of the future of the Senate in person. But he can add to the Senate and what he can bring."

The praise by the two Democrats follows praise by the White House shortly after Lugar conceded the Senate race on Tuesday. The praise by both the Senate Democrats and the White House is unusual given that Lugar was a member of the opposing party.