Democrats on the House Ways and Means Committee yesterday stepped up their attack on President Reagan's new tax-cut plan, but the President personally ruled out any further compromise efforts, drawing firm battle lines for the coming fight.

In a second day of hearings, the Democrats produced new figures charging that Reagan's proposal is tilted toward upper-income taxpayers and that those earning less than $10,000 won't get enough relief to keep pace with inflation.

However, Reagan, meeting with 14 labor union presidents -- including indicted Teamsters President Roy Williams -- declared flatly that "I've dug in my heels" on his tax-cut proposal and "I can't retreat."

The hardening of positions yesterday pointed to an all-out fight between the administration and mainstream Democrats when the committee begins serious work on the tax-cut measure, probably next week.

The panel is slated to resume markup sessions Tuesday, dealing first with Reagan's proposed business tax reductions and then going on to consider the tax cuts for individuals. However, there's speculation the schedule may slip.

Democrats are working on their own alternative plan designed to concentrate more of the tax relief in the $10,000 to $50,000 brackets, but so far they've been unable to agree on a formula. Republicans are backing Reagan's proposal.

The Democrats' attack yesterday was more aggressive and more broadly mounted than their previous criticism, reflecting a new boldness by members following the collapse of compromise talks with the White House last week.

Rep. Marty Russo (d-Ill.) charged the administration was dealing lower-income Americans a double blow -- restricting them to relatively small-sized tax cuts while at the same time cutting federal programs that benefit them most.

Democrats also chided the White House for refusing to accept their offer to go along with its three-year, 25 percent tax-cut plan on condition that the final installment of Reagan's economic goals.

"When you tell me that you are opposed to a trigger, you're telling me that you don't believe your own economic assumptions," Rep. Wyche Fowler (d-Ga.) told administration officials at the hearing.

The Democrats' new figures showed that while all taxpayers would enjoy some income tax cuts under Reagan's plan, the reductions for those in lower brackets would not be enough to offset inflation and rising Social Security taxes.

For example, while those in the $100,000-to-$200,000 group would wind up saving an average $2,840 per return after inflation and Social Security tax increases, those in the $5,000 bracket actually would have to pay $42 more.

Similarly, those earning between $20,000 and $30,000 would receive an average overall reduction of $103 after inflation and rising Social Security taxes, while those in the $200,000 bracket would get a cut of $20,300.

The figures, compiled by the congressional Joint Committee on Taxation, were made public by Rep. Donald J. Pease (D-Ohio). Pease said even if the Social Security tax hike were not included, poorer workers would be hurt.

However, Reagan administration officials were unmoved by the Democrats' criticism, countering that the president's proposal would merely parcel out of tax relief in proportion to the amount of taxes each income group paid.

John E. Chapoton, assistant secretary of the Treasury for tax policy, noted that taxpayers in the $20,000-to-$30,000 group, who pay 20.7 percent of all federal individual income taxes, would get 21.3 percent of the relief.

Those in the $30,000-to-$50,000 brackets, who pay 30.3 percent of taxes, would get a 31.8 percent cut, Chapoton added. He said the $5,000-to-$10,000 group, which pays 2.3 percent of taxes, would receive a 2.7 percent cut.

The Treasury official conceded that poorer taxpayers would end up "a little behind" after the impact of inflation and rising Social Security taxes is considered.

But he argued that the whole point of Reagan's plan is to avoid using tax cuts as a way to redistribute income from higher-income persons to poorer ones, as Democrats have sought to do in the past.

Echoing Reagan's stance earlier in the day, Chapoton said the administration was "locked into" the kind of cuts that would reduce tax rates by the same percentage in all income brackets.

Reagan's remarks to the labor leaders were relayed by the White House press office.