The Democrats regard their budget as a declaration of the party they have become in the fifth year of the reign of Ronald Reagan. The Republicans view their tax-simplification program as a blueprint of the party they hope to be.

The Democrats, beaten into the ground last November, are speaking in chastened tones in the budget that is going through the House this week. They hope it will prove to the country that they have learned from defeat and have abandoned a concept that was once sacred to them -- namely, that government can, and should, do everything for people.

For once, they have not shopped around for new constituencies to help. They have offered no new programs. And they have cut back old favorites, like Medicare.

House Democrats are tremendously pleased with themselves for having fashioned a document that, they believe, presents them as fiscally responsible, compassionate and better acquainted with reality than the Republican-controlled Senate. They expect their statement of lowered expectations to be the first step back to their former predominance.

The Republicans, riding high after Reagan's 49-state triumph, are about to present a major tax-simplification bill, one they think will lead them to a long-deferred encounter with "populism" that could transform them into the majority party.

What the Democrats like best about their manifesto is that it proves that while they are no longer big spenders, they have not lost their vaunted compassion. Their refusal to go along with the Senate freeze on Social Security cost-of-living adjustments proves, they think, that they are kindly economizers who, unlike Reagan, keep their promises to old people.

They expect applause for showing a profile in courage on defense spending. They have frozen Pentagon funds, with no allowance for inflation. But since the country has suddenly awakened on the deficit and has indicated a growing agreement with Rep. Pat Williams (D-Mont.), who calls defense contractors "desperadoes hiding behind their American flag lapel pins," it was not an act of supreme valor to settle for a $292 billion defense budget.

Still, when you remember that the Democrats fell apart so recently and capitulated to Reagan on the MX missile out of fear of being thought "weak on defense," the freeze represents a unity and resolve that the Democrats had almost despaired of recapturing.

The Republicans think that, in the long run, their tax proposal will beat the Democrats' budget as a crowd-pleaser. No Democrat could have written a fairer or more equitable proposal than Treasury I, as the original version was called. In that form, Republicans believe, it would have forever erased the image of the GOP as the patron saint of the country club and the executive suite.

The final form will be revealed by Reagan next week. The chipping-away has begun, led by Reagan and Rep. Jack Kemp (R-N.Y.), tax-reformer and would-be president, who has proposed a break for oil and gas companies.

The House budget was crafted, with considerable dispatch and flair, by Rep. William H. Gray III (D-Pa.), chairman of the Budget Committee. It preserves social programs for the poor but cuts into urban grants and Amtrak. The Democrats managed to keep student-loan, nutrition and jobs programs at current levels. Their caucus was reasonably harmonious and reflected none of the organizational and substantive strife afflicting the party everywhere else.

Although certain Republicans call their opponents' handiwork on defense a "white-flag budget," Democrats have had support and encouragement from Republicans for at least stamping a foot at the Pentagon. They note that the president is on the defensive about waste, fraud and abuse. They note also that the president, having called any defense cuts "irresponsible," accepted Senate surgery.

The Democrats are taking a little nourishment now. Their next job is to figure out how to handle the tax-simplification challenge. They cannot resist it; they understand that they will have to help Reagan achieve what could be his most notable and durable accomplishment -- one which could reduce them to minority status.

But they tell themselves that tax simplification has no chance unless Reagan barnstorms the country in its behalf. The special interests ranged against it, many of them Reagan's best friends, will fight tooth and nail. If he fails, he will be tagged as a captive of special interests. The Democrats know what that's like.