An 11th-hour compromise on crucial portions of the stymied Social Security bill was agreed to, then instantly started falling apart yesterday when tax-writing House Democrats rebelled against their leader.

A seeming agreement between Sen. Russell B. Long (D-La.) and Rep. Al Ullman (D-Ore.), leaders of Senate and House conferees, appeared less certain after Ullman met privately with House Ways and Means Committee Democrats.

The major sticking point, according [PARAGRAPH ILLEGIBLE] by the Senate, these included one amendment by Sen. Daniel Patrick Moynihan (D-N.Y.) to give states fiscal relief in the form of about $1 billion in extra welfare money next year.

Attempts to reach an agreement on deadlocked sections of the bill were intensified after Long suggested Monday that the conferees were so hopelessly deadlocked that final action might be put off until next year.

The Long-Ullman meeting led to a reconvening of the conference committee for this morning. Ullman, apparently counting on acceptance of the agreement, tentatively scheduled House floor discussion of the conference report for next week.

But after Ways and Means Democrats caucaused with Ullman yesterday. Some Capitol Hill sources said a continued deadlock in conference was likely.

Additional pressure on the conferences to settle difference was coming from the White House, where President Carter met yesterday with key advisers to discuss the Social Security deadlock.

Press secretary Jody Powell said Carter had received indicates from several individuals, including Long that progress was being made.

Powell said, "An effort is under way to resolve their differences and that's encouraging."

Passage of the legislation this year is considered important so that federal and private budget planners can determine income and expense amounts for 1978.

As discussions continued among House and Senate conferees, meeting separately yesterday, it became clear that the principal roadblock was the tuition credit.

The Senate on several occasions has voted overwhelming in favor of the credit to parents of college students and Long is considered to be under heavy pressure to retain it in the final conference bill.

But critics of the plan consider the credit, which at today's rates could cost the Treasury $1.2 million per year, an unacceptable windfall for middle-class taxpayers.

According to reports yesterday, Ullman's agreement with long would have had the House accepting a "minimal" tax credit, a step that critics said would surely lead to future expansion of the credit.

These some reports indicated that Long in return for House acceptance of the Senate's welfare provisions, would agree to go to conference early next year to work out differences on a House passed welfare bill.

Ways and Means Committee members have objected to inclusion of the Welfare proposals in the Social Security bill, arguing that separate and distinct issues are at stake.

Ironically, perhaps, the conferees appeared to be closer to agreement on the Social Security provisions of the bill that they were on tuition credit and Welfare.