A group of 40 conservative Democrats who may hold the balance of power in the House went President Reagan one better yesterday and urged him to propose an additional $11.2 billion in specific budget cuts.

The Democrats, led by Charles W. Stenholm of Texas, made their proposals to Reagan at a breakfast meeting. He promised to give them serious consideration, and added: "You've made my day."

But on Capitol Hill, in the second test vote of the season on Reagan's spending proposals, there was a contrary signal. On straight party lines, the House Post Office and Civil Service Committee opposed $1.3 billion in budget cuts sought by the president.

The committee voted against a $644 million reduction in the postal subsidy and a reduction of $655 million that would be achieved by giving cost-of-living increases in federal retirement benefits once rather than twice a year.

It reached no decision on the related question of recommended federal pay raises, saying that this should await further word from the administration. p

The votes came as the Post Office panel framed a report to the House Budget Committee estimating spending during the next fiscal year for programs under its jurisdiction.

Rep. Edward J. Derwinski (R-Ill.), the senior Republican on the postal unit, complained that if every committee "takes the position that spending cuts are needed but items under our jurisdiction are sacred cows" there won't be any reduction in federal spending.

Democratic leaders denied that the Reagan honeymoon is coming to an end, and House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) said, "We're not obstructing, we're going along."

But the committee action may have been an omen. The House Budget Committee also showed signs of partisan restlessness, calling the inspectors general of several agencies to testify on the "waste, fraud and abuse" Reagan claims abounds in government. Some of the Democrats on the committee say they doubt that the president has the specifics to back up his charges.

The additional budget cuts the group of 40 conservative Democrats proposed in the 1981 and 1982 budgets include private funding of the strategic petroleum reserve, excluding housing costs from the government index on which cost-of-living adjustments are based, reducing nonmilitary, nonstrategic foreign aid by 10 percent, repealing the Davis-Bacon Act, which requires a union wage scale on government jobs, and eliminating the Legal Services Corp., a favorite Reagan target when he was governor of California.

"I like this, I might consider becoming a Democrat again," Rep. G.V. (Sonny) Montgomery (D-Miss.) quoted Reagan as saying in response to the proposals.

Other developments yesterday on the president's proposed economic package:

The American Conservative Union announced that it will spend "whatever it takes" to identify and defeat members of Congress who oppose the Reagn economic plan.Rep. Mickey Edwards (R-Okla.) said the ACU will use television, radio and newspaper ads to let voters know what their representatives do, spending as much as $500,000, if necessary.

Secretary of the Treasury Donald T. Reagan gave businessmen a pep talk on behalf of the economic plan at the U.s. Chamber of Commerce, warning that the tactics of those who oppose it will be to "delay, delay, delay."

Senate Majority leader Howard H. Baker Jr. (R-Tenn.) estimated that Congress would act on the president's budget and tax reduction proposals by July 4. This appears to be an optimistic prediction, at least for the tax bill, which provoked a mixed response yesterday even from the group conservative Democrats who are so enthusiastic about the budget cuts.

One of these budget cuts, according to Montgomery, will be a slow-down in the construction of Veterans' Administration hospitals. Montgomery quoted Reagan as saying this at the breakfast, and also as saying that the administration would increase the interest rates on loans taken out against VA life insurance policies.

Leaders of the National Youth Employment Coalition launched a lobbying campaign against prospective efforts of the Reagan administration to cut back on job training programs.

Susan King, former head of the Consumer Product Safety Commission, said the 30 percent budget cut proposed for the agency "would cripple the commission's efforts to carry out its mission."

Transportation Secretary Drew Lewis told a congressional committee that he will not follow the recommendations of the Office of Management and Budget calling for substantial increases in Amtrak fares.

"We don't plan to follow OMB's program on how to run Amtrak," Lewis said.

At the White House the president prepared to hold his second news conference at 2 p.m. today in the Old Executive Office Building.

For the first time, after two opening questions from wire service reporters, the order of the questions is to be determined by a lottery. Each reporter who attended the daily White House briefing yesterday was given a card to sign, and the cards were drawn later in the day by the president from one of the many jelly-bean jars he has around the White House.

The results of the drawing were unpopular with the television networks, because no network was higher than 40th on the list. Twenty-two questions were asked at Reagan's first presidential news conference. CAPTION: Picture, President Reagan, flanked by Reps. Charles W. Stenholm and G.V. (Sonny) Montgomery, laughs during a breakfast meeting with conservative Democratic members of Congress, who asked Reagan for $11.2 billion in specific budget cuts. By Frank Johnston -- The Washington Post