No one tells Ted Kennedy what to do; in any case, the Senate’s Democratic leaders were fine with his plan to give a big speech two days before President George W. Bush announced a troop “surge” in Iraq. They are generally glad to let Kennedy play the role he relishes: Irish-American Isaiah, calling his party to account even as legislative insiders keep their distance.

This time party brass got more than they bargained for. Summoning the authority of his years as an intimate witness to history, Kennedy made an eloquent case for a Senate vote on the surge and for a court test of its legitimacy under the War Powers Resolution. “Iraq is George Bush’s Vietnam!” he thundered. “Echoes of that disaster are all around us today!”

It was, in its own way, a defining moment. He got a standing ovation and, the next day, congratulations all around on the Hill. By the end of the week–in the aftermath of Bush’s tepid speech and Condi Rice’s evasive testimony–Kennedy looked prescient.

A generation ago, a war–Vietnam–launched a realignment of American politics. Now, it seems increasingly clear, Iraq is doing the same. In 1968 college students flocked to the New Hampshire primary to protest Lyndon Johnson’s policies, sparking a civil war in the Democratic Party on foreign policy that lasted for a generation. By contrast, Vietnam united the GOP around an anti-communist crusade that endured for decades. “Ronald Reagan was gung-ho about Vietnam,” says Craig Shirley, a GOP operative and Reagan biographer. “It solidified his world view, and the party’s.”

Now a mirror image is developing. Democrats seem to be uniting around a theme–the primacy of global diplomacy and congressional review. Republicans, by contrast, have lost the unity that they had during the cold war and the early years of the war on terror.

As Republican divisions grow, Democrats, pressed by their antiwar grass roots, are drawing together. Except for “Independent Democrat” Sen. Joe Lieberman, Dems are increasingly of one mind about Iraq in particular and antiterrorism strategy in general. A vote on surge spending–which Democratic Senate leaders had hoped to avoid and which is technically difficult to devise–now is likely at some point. In general, the party seems less fearful of the old “soft on defense” shibboleth, and ever more tolerant of groups such as Win Without War and Move On. One of the Senate’s few other hawkish Democrats, Sen. Evan Bayh, told me that he opposes the surge, and agreed that Congress might have to face the question of funding at some point. The Senate’s growing ranks of Democratic presidential contenders–Chris Dodd jumped in last week, Hillary Clinton and Barack Obama are expected to do so soon–are gravitating toward a bring-them-home-quickly stance. “We don’t want to come off looking like wimps,” said Terry McAuliffe, a Clinton supporter and former party chairman. But he added: “We’re jumping all over ourselves now to see who can be the toughest on Bush and the war.” It’s a fateful competition–which Ted Kennedy already won.