President Reagan's parting budget presents George Bush with an early test of political leadership as he seeks to fulfill campaign pledges of budget austerity and government humanity without raising taxes.

Leaders of the Democratic-led Congress say they will give Bush the benefit of the doubt on his "flexible freeze" approach to budgeting, in which increases in spending above the rate of inflation must be offset by accompanying decreases in other areas.But at the same time the Democrats are pointing out the distance between Reagan's $1.15 trillion spending blueprint and Bush's campaign promises.

Bush's aides are letting it be known that he will revise Reagan's work within weeks.

Reagan's farewell budget, calling anew for expanded military spending and contracted social programs, "provides a formula for a meaner, harsher America," said Sen. James Sasser, D-Tenn., chairman of the Senate Budget Committee.

And House Speaker Jim Wright, D-Texas, said the Reagan budget "obviously does not respond to the promises made by President-elect Bush during the campaign. . . . Obviously, this is not a kinder, gentler budget."

Those "kinder, gentler" programs promised by Bush include large proposed increases for education, for the Head Start program, for expanded Medicaid payments to low-income pregnant women and their babies, and for environmental cleanup.

Vice presidential chief of staff Craig Fuller was asked Monday at a news briefing whether Bush, as president, would continue to support such programs, and wouldn't they cost the government a lot of money it didn't have?

"Yes and maybe," Fuller responded.

The Bush camp was trying to keep above - or at least out of - the fray as Reagan's final spending plan went to the Hill, where it seemed guaranteed to be ignored as congressional budget writers await the Bush amendments.

The vice president is expected to outline his modifications within a few weeks after his inauguration on Jan. 20, most likely in an address to a joint session of Congress. However, aides say the exact format of his budget submission has not yet been decided.

It's still "too early to say" how much of the Reagan budget Bush will keep and how much he'll discard, said Nicholas Brady, who is Treasury secretary now and will occupy the same post in the Bush administration.

"It is certainly possible that much of the current budget will remain on the table," Brady said. However, Fuller observed, "there are surely going to be variances."

During Bush's campaign, he often seemed to be having it both ways politically: seeking to bask in Reagan's popularity as loyal vice president for the past 71/2 years while distancing himself from some of the more unpopular Reagan deeds and policies.