Sunday's vote in the House of Representatives is huge for Barack Obama. He found his voice on healthcare – after the special election in Massachusetts took away his filibuster-proof Senate. He took an issue that seemed dead, a party in disarray, and an administration on the defensive and rallied them.

President Obama has managed something that eluded Roosevelt, Truman, Kennedy, Nixon, Carter, Clinton, and both Bush Sr and Bush Jr. Winning national health insurance – even this weak version – will be his legacy. And it restores his reputation as an effective and eloquent force. Even more important, he avoids a loss that would have diminished his star to the vanishing point. Lose this and he becomes Jimmy Carter. Win it and he does what even the greats could not do.

And here's something people are missing: this was the Democrat's signature issue. It has been since 1945 (when FDR designed a reform and left it as a legacy to Truman who made it the cause of his life). When was the last time a Democratic president rallied his party on one of their issues and overcame fierce Republican opposition? Bill Clinton (1993-2001) did not manage it once; he won essentially Republican issues – budget balance, Nafta, welfare reform. Jimmy Carter (1977-81)? No, again. To find a Democrat winning a contested victory on a Democratic issue you have to go back to Lyndon Johnson before the 1966 midterm. Forget the politics for a moment. On the substance, I'd consider this the biggest Democratic victory in 45 years.

And the midterm test that everyone is focused on now is shortsighted. Lyndon Johnson changed America with the Civil Rights Act (1964) and Medicare (1965). No Democrat and few Republicans would mess with those laws. Lyndon Johnson usually ranks as a near great president – and without Vietnam he'd rank even higher. But he got buried in the 1966 midterm election. For people with an eye to history, the midterm test is not a reliable one.

Is the health reform good for Republicans? No. They already have the anti-government crowd sewed up. Before the vote they could make two arguments: 1. Democrats can't get anything done; 2. And what they want to do is bad – too much government. Now, they lose the "Democrats are ineffectual" argument. As to the second argument, the Dems have a chance to prove them wrong with an actual programme.

But there's one final point that no one should miss. The politics continues. Democrats have a chance to reframe this debate and take their momentum on to other issues. Republicans will try to keep portraying Democrats as wild and out of control. But look at how zany the politics have been. The Republicans dominated that debate for a year. Then, crazily, they won a long-shot special election – and promptly lost control of the debate. Nothing became Obama so much as his response to a stinging defeat. What's next? Here's the irresistible conclusion: expect the unexpected.