President Bush is revealing his budget in bits and pieces in advance of its release Thursday, as he tries to emphasize those parts likely to engender a warm reception on Capitol Hill.

"I am under no illusions that we are going to keep everybody happy," he said while asking GOP senators for a sympathetic hearing in his speech to Congress on Thursday night. He repeated the message in a series of stops in the Capitol on Tuesday, but specifically mentioned those things the lawmakers wanted to hear - no tax increase, and more money for the environment, education, and fighting drugs."It will make I think a strong beginning in some of the areas that a lot of us talked about in the past campaign," he said.

But the Bush plan necessarily will include many less popular items, many left over from the fiscal 1990 spending plan drafted by President Reagan - such as a $5 billion reduction in the Medicare health plan for the elderly. And Bush has decided to include some controversial items of his own as he struggles to balance his no-tax pledge and other campaign promises.

Statements by Bush himself, and conversations with administration and congressional officials, reveal the list of his proposals will include:

-A cut in the tax rate on capital gains, as promised by Bush in the campaign. The administration will claim the lower tax rate actually boosts government revenues, an extremely controversial stand especially among Democrats who oppose the cut as favoring the wealthy.

The rate, now 28 percent, would be reduced more for longer investments in order to emphasize business investment rather than fast-buck transactions.

-Holding the military budget to only an inflationary increase in the fiscal year beginning Oct. 1, and then only slightly more than inflation in the next two years. Conservatives are sure to oppose this sharp retreat from the Reagan military build-up.

Reagan had proposed increasing military spending authority to $315.2 billion next year, and Bush's stance would cut that $6 billion. Actual cash outlays, money spent during 1990, would be reduced $2 billion.

-A child-care tax credit, instead of the more direct federal child-care system favored by many in the Democratic Congress. The tax credit could eventually cost the Treasury about $2 billion a year. At much smaller cost, Bush will call for an expanded tax benefit for adoption.

Bush on Tuesday declared his budget "pretty well finalized," talking to reporters at a final budget session with budget director Richard Darman, Treasury Secretary Nicholas Brady, economic adviser Michael Boskin and Vice President Dan Quayle.

His initiatives, plus his savings-and-loan bailout plan which will cost taxpayers an estimated $1.9 billion next year, will be included in a 125-page proposal to Congress Thursday. The proposals in the document will be layered upon the $1.15 trillion budget that Reagan submitted.

Although Bush is changing its emphasis somewhat, he relies on the Reagan program to reduce the deficit below the $100 billion maximum required by the Gramm-Rudman law.

Reagan's economic and technical estimates, considered by many in Congress and the private sector as too optimistic, are largely embraced. Bush also goes along with Reagan's call for eliminating many domestic programs and cutting many others, including farm subsidies.

Although Bush rejects Reagan's proposal to cut the Medicaid program for the poor, and plans to increase those benefits modestly through administrative cost-cutting, he plans to include his predecessor's $5 billion cut in the Medicare program's expected spending.