In a sign that Democrats are now circling the wagons around Barack Obama as the party's nominee, Joe Biden has written a guest column in the Wall St. Journal rebutting an earlier piece by Joe Lieberman, which attacked Obama's credibility on national security. Biden has not formally endorsed a candidate in the race, but his public defense of Obama is telling.

"Sen. Obama is right that the U.S. should be willing to engage Iran on its nuclear program without 'preconditions' - i.e. without insisting that Iran first freeze the program, which is the very subject of any negotiations," Biden writes. "He has been clear that he would not become personally involved until the necessary preparations had been made and unless he was convinced his engagement would advance our interests."