The Senate has taken another step toward approving a massive health care bill. A majority of Senators approved an amendment making final changes to the bill Tuesday morning.

As the Senate lurches towards a final vote on its health overhaul bill, some people are daring to look ahead to the last step in the painstaking process: marrying the Senate and House bills. Like everything else about this bill, putting the two measures together won’t be easy either.

On the surface, the House and Senate health overhaul bills have a lot in common.

They both require everyone to have health insurance and provide generous subsidies to help people buy it. They both create new insurance “exchanges” where individuals and small businesses would go to buy insurance, and they both put new requirements on health insurers, like banning them from discriminating against people who have pre-existing medical conditions.

But the bills differ in some profound ways, too – particularly in how they would be financed.

“The biggest challenge is getting the money sorted out,” says Alec Vachon, a health policy consultant and former Republican Senate health staffer.

Both bills rely for about half their funding from reductions to the Medicare program.

But for the other half, Vachon notes that the House bill relies largely on raising money from wealthy individuals, “The surcharge on singles making over a half a million dollars and couples making over a million bucks.”

While in the Senate bill, Vachon says, “They have industry tax increases that don’t appear in the House bill.”

Like taxes on drugmakers, medical devices, and health insurance companies, as well as a separate and controversial tax on generous health insurance plans, known as the Cadillac health plan tax.

But Vachon says he thinks Democrats are more than motivated to work things out. And, because the Congressional Budget Office says the Senate bill would actually reduce the deficit, negotiators have some financial wiggle room.

There’s nothing here that’s not a matter of negotiation,” Vachon says. “So maybe they can dial back on the Cadillac tax – it will grow more slowly. Again, they’re going to do that cross the board, and all the savers, all the new revenue raisers, they’re gonna tweak things. But they do have the money there and they do have the will, and those seem to be the two things that are important.”

Perhaps the most difficult issue to reconcile will be abortion. Everyone says they want the bill to reflect the status quo – no federal funding of abortion. But finding language that does that and no more has proved difficult.

The House bill includes an amendment offered by pro-life Democrat Bart Stupak of Michigan and supported by anti-abortion groups. It would bar federal abortion funding and prevent plans that receive federal subsidies from paying for abortions.

The Senate bill’s abortion language was negotiated primarily by pro-life Democrat Ben Nelson of Nebraska. Like the House bill, it would also bar direct federal abortion funding.

It would, however, let private plans in the new insurance exchanges offer abortion coverage, as long as people write separate checks each month; one for the abortion coverage and one for all other benefits.

That doesn’t work for abortion-rights groups.

“Who in their right mind would ask somebody to write two checks for their health insurance?” asks Laurie Rubiner, Planned Parenthood’s Vice President for Public Policy.

“You wouldn’t ask anybody to do that for any other aspect of their health care. Can you imagine asking somebody to write a separate check for their coverage for diabetes or heart transplant or any other medical procedure? You never would do that. ”

But unlike the House bill, the Senate abortion language has also drawn opposition from anti-abortion groups.

Tom McClusky, vice president of Family Research Council Action, says “When you’re writing two checks but drawing from the same bank, it’s not really a very good accounting system. And government is still funding abortions.”

So groups on both sides are worried about what might happen in a House Senate conference.

McClusky says he’s worried about something else, too – the fact that the leadership in both the House and Senate support abortion rights.

“I’m sure that what they’re thinking is: Since you have two different versions of abortion language, they would feel free to do a third version themselves, possibly reverting back to either the House or the Senate’s original language, which did fund abortion.”

But while abortion may be the hardest issue, there are other policy differences, too. Among them is the fact that the House bill includes a government-sponsored public option and the Senate bill doesn’t.

But House negotiators will need to remember one important thing: The Senate’s margin for error is zero, So any single Senator who says no can stop the entire effort in its tracks.