On Sunday, March 22, one of the Democratic Party's most astute backroom operatives was jolted into reality. Ronald Reagan, the genial old gipper, intends to destroy the 50-year-old welfare state.

What jolted him was the sight of budget director David Stockman, answering the last question on ABC's "Issues and Answers." Asked about Reagan's proposed end to federally financed legal services for the poor, Stockman replied:

"I don't believe that there is any entitlement, any basic right to legal services or any other kind of services, and the idea that's been established over the last 10 years that almost every service that someone might need in life ought to be provided, financed by the government as a matter of basic right, is wrong. We challenge that. We reject that notion."

The shock effect on the Democratic operative produced a political reaction.

"If that's what they are doing to our country," he told us a day later, "then it's open war." His idea was to open fire, not on Reagan's social welfare cuts, but on his dispatch of a few military advisers to El Salvador. While this operative concedes El Salvador is no Vietnam, he hopes it is an exploitable weakness.

During the two weeks before Stockman's blunt remarks, Democrats in Congress came to the same conclusion as the political operative. They had discovered the Reagan budget's impact is not in its numbers, relatively small in the trillion-dollar economy, but in its design to steadily reduce the pervasiveness of government.

Stockman's candor signaled that the Reagan honeymoon is truly over. Although popular enough to deter frontal assault, the Democrats have concluded that Reagan's similarity to Dwight D. Eisenhower's placid moderation, so striking in style, does not extend to substance. While Richard M. Nixon's rhetoric was radical and his policy moderate, Reagan takes the opposite tack.

Stockman's harshness is not Reagan's style and, in fact, has been privately criticized by White House aides. But his stated desires do not deviate from Reagan's wishes.

While some Democratic practitioners talk about a hard line on the poor being "unseemly" amid the Reagan inaugural opulence and round-trip jet flights to the ranch at Santa Barbara, they are not confident enough of this approach to go public. In truth, the Democratic consensus on Capitol Hill is that Reagan's policy maintains a high margin of popularity.

Thus, the size of the Reagan budget cuts are under no real assault in Congress. The effort to instead reduce "tax expenditures" (a euphemism for increasing certain taxes) has no real chance for approval. While Rep. Dan Rostenkowski, chairman of the House Ways and Means Committee, has declared Kemp-Roth tax reduction dead, he knows he cannot tell his blue-collar constituents in Chicago that there will be no individual tax cuts this year. The Kemp-Roth label may be gone, but the idea lives.

Since a frontal attack on Reagan's grand design cannot succeed, a flanking assault is necessary -- perhaps traveling through Central America. Secretary of State Alexander Haig's derogatory comments on the murdered Maryknoll nuns in El Salvador are seized upon by Democratic activists in hopes of energizing the Roman Catholic Church against Reagan.

This also explains the extraordinary interest of certain Democratic operatives in the mountain of gossip about alleged sexual promiscuity by prominent Republicans, only the tip of which has become public at this writing. "That could wreck Reagan's economic program," a hopeful Democrat predicted to us in explaining his backstage efforts to publicize the rumors.

Such flanking attacks are considered necessary by Democrats, considering the scale of Reagan's grand design. Rep. Henry Reuss of Wisconsin, ending his long and distinguished congressional career as an unreconstructed liberal, sees the Reagan program as "dismantling our tax system." Rep. Barney Frank, the brilliant first-term liberal from Massachusetts, sees block grants to the states as "a way station" en route to no government help at all. Unlike budget cuts, tax reduction and block grants are permanent changes that help transform Stockman's wishes into reality.

Reuss, Frank and other perceptive Democrats believe Dave Stockman means what he says and says what Ronald Reagan means. Many Democrats also fear the American people might agree with them. So immense are the stakes for what kind of country this is to be that civility in the attack on the president's program is declining as post-honeymoon politics begins.