President Carter's welfare overhaul hit a snag yesterday when Rep. Al Ullman (D-Ore.), chairman of the powerful House Ways and Means Committee, disagreed with major elements of it and urged the President to delay unveiling the package.

White House sources said the President seemed to be leaning toward his self-imposed Friday deadline for announcing his proposal, but they did not rule out a possible delay.

Press secretary Jody Powell said the President had made some "tentative conclusions" but would make "no final decisions" until he meets today with Sen. Russell B. Long (D-La.) chairman of the Finance Committee, Long is also expected to raise objections and request a delay to iron out major differences.

Chances for congressional passage of the plan in its present form are highly unlikely if the chairmen disagree as strongly with some of the proposals as Ullman indicated yesterday.

Ullman asked Carter to revise a proposal that bases welfare payments to employed recipients on the size of the family, as well as earnings. Payments to low-income working recipients should be based solely on earnings, Ullman argued.

Health, Education and Welfare tables in the administration's welfare package, obtained by Ullman this week, spell out the federal payments for those expected to work.

The tables show that, in a family of four with one parent working and earning $4,000, the annual federal cash payment plus an earned-income tax credit would amount to $2,600 - making the total income $6,600.

For each additional family member, the federal payment . . . tax credit would be increased $600. Therefore, for a family of seven making $4,000 the total income would be $8,400.

The earned-income tax credit, which reduces taxes for low-income people, now phases out completely at $8,000. The administration would phase it out at a much higher income, about $16,000, sources said.

The administration has based benefits on family size for both those who are working and those who are not expected to work (such as women with dependent children), it does not want working families to fall behind welfare families in total income.

Ullman supports welfare payments to unemployable persons but insists that those who are expected to work and can find jobs should be moved "out of the welfare syndrome into the job syndrome. When you're on a job you don't get paid according to the size of your family," he said.

Various options in the welfare package have been criticized by such groups as mayors and housing officials over the past several weeks, but Ullman's was the first public criticism from a congressman whose support could be crucial to enactment of the program.

Ullman did say, "I'm in full accord with much of what they are recommending."

Administration sources emphasized that they were taking Ullman's criticisms seriously.

HEW Secretary Joseph A. Califarno Jr., who attended the meeting with Carter and Ullman, later spoke with Ullman and others on the Hill to see if there was any way to resolve the problem by Friday, sources said.

An aide to Califano would not comment except to say, "We always take anything the chairman of Ways and Means says seriously."

Given the complexities of the package, one welfare expert seemed doubtful that much could be done to make constructive changes before the President's scheduled announcement.