THE CONGRESSIONAL Black Caucus last week accepted Ronald Reagan's invitation to Democrats to come up with their own budget if they didn't like his. Not suprisingly, the black legislators' plan has no tax cut and saves $27.4 billion for social programs by dipping into the defense budget instead of poor people's pockets.

With Democratic party leaders staying quiet on much of Reagan's plans as they pass him yards and yards of rope, with a majority of public opinions on Reagan's side, at least for a time, and with the old guns versus butter mindset in disrepute, the caucus' budget probably will go nowhere, of course.

But it's significant because the caucus proposal hints at the potential anti-Ronald Reagan coalition composed of all those -- poor and middle class -- who will feel the budget bite, and whose lives and children's lives will be changed because of it.

The black congressmen's motivations to throw their support behind issues of importance to people beyond their predominantly black city constituents isn't all noble. With upcoming redistricting based on new census figures that show fewer city residents, they know many of them will end up in new districts with more whites and middle-class residents. But the proposals reminds us that besides poor people, a lot of not-so-poor folks who believed Ronald Reagan was on their side also are going to lose something under his stewardship.

The caucus' renegade budget is aimed not-so-subtly at the white middle class.It restores funding that provides a chance for home ownership to millions of Americans, student loan funds for college students, a rehabilitation loan program for low-interest loans frequently used by middle-income people and the low-interest loans for people wishing to do energy conservation on their homes -- all of which would be wiped out if Reagan wins the day.

Caucus Chairman Del. Walter E. Fauntroy (D-D.C.) and company argue that the country can increase the social programs for the poor and beef up defense without ruining the economy. But they also argue that the economy will not necessarily be ruined if the middle class does not suffer reduced support to symphony orchestras, ballet, summer entertainment programs and withdrawn tax credits to nonprofit institutions, as Reagan proposes.

Why, they argue, must the average citizens have to endure dirtier state parks because the jobs of the people who keep them up will be eliminated? Remember, though, it is not Walter Fauntroy's city constituency that spends its summer cruising the nation's state parks. And that is something more and more Reagan supporters will come to realize.

So in coming up with an alternative budget, the caucus took issue with the administration on behalf of an awful lot of people who want to trim runaway or ineffcient programs and reduce inflation, but who will agree less with Reagan when they come to understand the role of the federal dollar has played in giving them or their family a leg up in the world or allowed them to enjoy a few days in a clean state park.

Politically, it makes sense for the caucus to broaden its appeal, to find ways to attract white voters. If not, a bunch of them may be out of office.

Some people argue that the caucus, in taking on the administration, has less to lose than the rest of the Democratic Party leadership within or without the government. They don't have to be as cautious as Ted Kennedy or Pat Brown or Walter Mondale who are shooting for higher stakes.

They don't even have to be as cautious as Democratic county chairmen across the country who must live with the more conservative tendencies of today's grass-roots voter.

Yes, Walter Fauntroy doesn't have to worry much about the views of the Moral Majority or even those of an auto worker who wants to keep more of his $25,000 a year for himself.

But the caucus knows that the issues Ronald Reagan rode into the White House actually transcend race. And that the guy saving to send his kid to State U. or the fellow planning to drive his Winnebago to Sandy Point State Park has a lot in common with the woman who feeds her kids with food stamps.