For all the trouble House Democratic leaders are having rounding up enough votes to pass a major health care reform bill, the possibility they can pull it off cannot be ruled out, even at this late stage.

The 52 so-called "Blue Dog" moderate Democrats certainly hold the balance of power on the issue, but Speaker Pelosi only needs to pick up about 15 of them to ram the bill through.

That explains why House leaders are watching the Senate so carefully in the hope that the Finance Committee can reach some sort of a compromise that would have a chance of passing the Senate. That will help convince Blue Dog Democrats that they aren't being asked to cast a tough vote on a bill that is going nowhere in the Senate, as happened on the controversial cap and trade energy bill.

Remember, members are being asked to overhaul drastically about a sixth of the U.S. economy — a health care system which large majorities of Americans say they are personally satisfied with, despite worry about rising costs. Such big changes in social policy in America tend to be made by overwhelming bipartisan vote.

Social Security passed the House 372 to 33. Thirty years later, Medicare passed 313 to 115. Such margins give the members political cover in case the changes prove unpopular, and make it far less likely the changes will be soon undone. But no such margins are available for this bill.

Brit Hume is the senior political analyst for FOX News Channel.