The debate over the health care overhaul has moved behind closed doors.

After difficult votes in both the House and the Senate, Democratic leaders are eager to avoid a bipartisan conference that would give Republicans further opportunities to try to delay or kill the legislation.

Democrats are now trying to "ping-pong" a compromise by informally shuttling the legislation back and forth.

It's likely that the Senate version (HR 3590 ) will be amended with the final changes and put up for a final vote.

But compromise won't be easy.

Though the Senate has another $30 billion to spend before reaching Obama's $900 billion limit, a number of senators have warned that any substantive changes could cost them crucial votes.

Among the disagreements:

Public option. With the government-run option for health insurance left out of the Senate bill and likely the final compromise, several House Democrats are seeking additional spending to reduce the cost of insurance for the middle class.

Abortion spending. The House version bans using federal money for most abortions, while the Senate version maintains existing abortion funding rules. This has emerged as a major sticking point, with advocates on both sides threatening to vote against reform.

Taxes. Both versions include taxes that the other chamber opposes. The Senate bill would tap indoor tanning salons and high-cost "Cadillac" insurance plans, while the House version would dun wealthy households.

Insurance exchanges. Both versions of the bill include an exchange where individuals can shop for insurance. The House bill creates a single national exchange; the Senate version gives states more say by creating statewide exchanges.

Illegal immigrants. The House version lets illegal immigrants buy insurance on the exchange as long as they use their own money, while the Senate version bars them from the exchanges altogether.

Medicare Part D's donut hole. Both the House and Senate have tried to curry favor among senior citizens by closing the donut hole Medicare leaves in prescription drug costs. The House bill would eliminate the gap entirely by 2019, while the Senate has offered a $500, one-time reduction.

The Young Invincibles. Both plans let young adults, the nation's most uninsured age group, stay on their parents' policies longer. But the Senate version also creates a high-deductible option for people under 30.

Because the Senate passed its bill with more fragile alliances, the House may have to sacrifice more in negotiations. Democrats are aiming to deliver a bill to the President before his State of the Union address in mid-January.

"This has been put together very carefully," Sen. Ben Nelson (D-Neb.) said after the Senate passed its bill last month.

Ambreen Ali writes for Congress.org.