As the most crucial day of his young presidency approaches, Barack Obama is privately confident that he will prevail on healthcare reform. He has every reason to be.

The remarkable thing is that Mr Obama's healthcare reform has taken so long, drained so much energy from the majority party, and will pass (if it does) with almost no votes to spare. If the bill falls short, of course, Mr Obama will be forced to rebuild his presidency, much as his Democratic predecessor, Bill Clinton, had to do after his healthcare debacle in the early 1990s.

But even if he does prevail, as he should, the win will be at some cost. Many Americans see the Democrats as ramming reform down their throats despite many polls showing most people do not like the bill.

Still, when a Democratic president demands legislation from a Congress heavily controlled by his party, it is very rare that he doesn't get it. The party leaders will twist every congressional appendage to deliver, even if that means making some members walk the plank in the November elections. They are helped by a bitter memory. When the Clinton reform package dissolved, Democrats paid a huge price – a Republican Congress elected in 1994 that lasted for a dozen years. A few congressmen may have to take one for the team.

The Democrats hope that passage of healthcare will energise a lethargic party base. Low turnout among minorities, the young, and urban voters will doom many Democratic candidates this year.

The party hopes that a promise kept will make the losses less painful. But for some of the 41 members running for re-election who represent districts carried by Republican John McCain in 2008, an affirmative vote on healthcare could end their careers. President Obama will do his best to sell the bill after the fact; he thinks he can turn around public opinion by November. That is an optimistic assessment from a self-assured man.

More likely, Republicans – on their way to a considerable mid-term victory – will begin a campaign for repeal of the bill. Since it will take an all-GOP government to achieve this, the effort will last for years. That suits Republicans just fine since it will motivate their supporters to vote in multiple elections. But what if Americans come to like the new health arrangement? Then Mr Obama and the Democrats will have won the biggest gamble they have taken in decades.

The writer is director of the University of Virginia's Center for Politics