Joe Biden was going to be John Kerry's secretary of State. "That was what we were led to believe" before Kerry lost to George W. Bush in '04, says an aide to the Delaware senator, who is chairman of the Foreign Relations Committee. Now Biden, who has been to foreign policy in the Senate what Ted Kennedy has been to domestic policy (almost anyway!), is emerging as a major consigliere to Barack Obama—perhaps with his eye on State once again. Among the top items on Biden's agenda: making sure that Obama has better luck in November than Kerry did. That means, first, relentlessly attacking and counterattacking the Republicans on the campaign trail, especially on national-security issues. And, second, relentlessly defining John McCain as "joined at the hip" to Bush, as Biden put it in a speech in Washington on Tuesday.

In an interview, Biden said that he and other leading Democrats are "absolutely, thoroughly, totally" making a conscious effort to ensure that Obama doesn't become the next Kerry. The failure of Kerry and the Dems of '04 to seize control of the national-security agenda and counterattack Bush was a mistake "that was emblazoned in my mind," Biden says. Hence this week's onslaught of Democratic ripostes to Bush's seeming suggestion, in a speech to the Israeli Knesset, that Obama was guilty of "appeasement" for indicating he would negotiate with Iranian President Mahmoud Ahmadinejad and other U.S. adversaries. Biden called Bush's remarks a "long-distance Swift Boat attack" and said that the president seemed unaware that his secretary of Defense, Bob Gates, had "the day before" called for engagement with Iran and that Bush had previously "struck a deal with Libya's [Muammar] Kaddafi and wrote polite letters to North Korea's Kim Jong Il."

On Friday, Biden will fire back at another McCain proxy, Sen. Joe Lieberman, on the pages of The Wall Street Journal. In a Journal op-ed piece Wednesday, Lieberman repeated the appeasement charge against Obama, saying he has "not been willing to stand up to his party's left wing" and that the Democratic Party had drifted away from "Franklin Roosevelt, Harry Truman and John F. Kennedy," succumbing instead to "the old voices of partisanship and peace at any price." Though the Democrats have been divided throughout the primary season, attacks like these may help bring them together. Biden was among the first Democratic foreign-policy leaders to champion Obama aggressively—though he has not officially endorsed anyone.

So is Biden running to be President Obama's secretary of State? He denies it, saying he's worked with Hillary Clinton "much longer" than with Obama. That's a politician's non-answer, of course. To a striking degree, Biden and Obama seem to be achieving a mind meld on foreign policy. While some differences remain—Obama is more forthrightly opposed to future Iraq funding than Biden—both senators have been out in front calling for two brigades to be moved from Iraq to Afghanistan. And in his speech Tuesday, Biden echoed Obama's much-quoted effort to define down success in Iraq during testimony last month by Gen. David Petraeus and Ambassador Ryan Crocker—when the Democratic candidate suggested it was unrealistic to stay in Iraq until every vestige of Al Qaeda or Iranian influence was wiped out. The similarity was no accident, Biden indicated. "I discussed with Senator Obama how to proceed with Petraeus and Crocker," Biden told me. "He asked for my advice." Biden adds that one condition of his taking the State job would be that he doesn't turn into Colin Powell—someone marginalized by taking a different viewpoint than the president. "I wouldn't stay in a Powell role," he said. "I would want to make sure I was on same page as the president."

Obama campaign advisers say that the Illinois senator does seek out Biden's counsel quite often. While the two don't have a "warm personal friendship," says a Biden aide, their relationship has "evolved dramatically" from the early days when Biden watched with some skepticism as Obama rocketed to national renown and eclipsed his own presidential ambitions, as well as those of other more senior senators. "I can tell you that Senator Obama has the highest respect for Senator Biden," says Obama foreign-policy coordinator Denis McDonough. "It's obviously vital to have an open line to somebody who has been a key player in critical national-security debates for some time." Adds Obama campaign spokesman Bill Burton: "At this point there's no list [for secretary of State], but when one is drawn up Senator Biden would be on anyone's short list."

Obama, of course, has made his early opposition to the war in Iraq a key talking point; it's a comfort to him that Biden, though he voted for the Iraq War resolution in 2002, was aggressive in urging that the Democratic Caucus "had to take its time on Iraq debate, and couldn't just let president dictate the timing of it," says one Obama adviser. "That whole summer, and in the fall, he said we've got to make sure we kick tires on this. He is a real pro."