Washington ? You may think Democrats and Republicans are at odds over health care. Well, they’ve got nothing on House and Senate Democrats going after each other.

The intraparty disputes may prove the most grueling test of all as Congress tries to write a bill that fulfills President Barack Obama’s goal of extending coverage to millions of Americans and reining in rising medical costs.

The disagreements extend well beyond whether or not to allow the government to sell insurance in competition with the private market, though fissures over the so-called public plan — preferred in the House, less so in the Senate — have drawn the most attention.

Some of the toughest fights loom over what requirements employers should have to shoulder to see that their workers are covered, and perhaps stickiest of all, how to make coverage affordable and pay for extending it to millions of uninsured.

Senators would tax high-value health insurance plans to pay for covering the uninsured, an approach supporters say would curb health costs because it would lead to employers offering less generous benefits. The more populist House would tax the highest-income people, placing the burden of caring for the neediest Americans on the backs of millionaires.

“I don’t know how you split that difference,” said Rep. Artur Davis, D-Ala., a member of the House Ways and Means Committee. “It’s not just about numbers. These are philosophical differences about how you pay for reform.”

Any showdown between the House and Senate is a ways off, and will happen only if both succeed in passing their own health bills. Democratic leaders in both houses are working to finalize their legislation — a process that is itself fraught with difficulties — in time to hold floor debates within the next several weeks.

Presuming the House and Senate do pass bills, they will go to a conference committee made up of Democratic leaders and key committee chairs from both chambers. There, with plenty of input from the White House, the most powerful members of Congress will fight it out in private.

Senate Majority Leader Harry Reid, D-Nev., put it mildly this past week: “There are different views in the House,” he said.

Democrats are increasingly optimistic about the possibility of success on health legislation, and they’re quick to point out the many areas where the House and Senate legislation overlap.

Even within the commonalities are differences that will have to be worked out in what are sure to be tough and lengthy negotiations. But the big sticking points are government insurance, employer responsibility and paying the bill.