One of the most surprising developments of the 97th Congress has been the emergence of a famous "neo-conservative," Sen. Daniel Patrick Moynihan (D-N.Y.), as a leading neo-liberal critic of the Reagan administration.

Would you have believed a few months ago that Moynihan would end up as the choice of the liberal-oriented Democratic congressional leadership to make the party's 8 reply to the president's televised pitch last week for his tax program?

Yet, only two hours after Reagan finished, there was Moynihan also on the networks firing away at Reagan's arguments. Even before going on the air, however, the senator was already saying, "Something like an auction of the Treasury has been going on. This administration is seemingly willing to pay any price to win votes for their version of the tax cut, simply to gain a victory on their own terms."

The honor of speaking for the Democrats, though, was not bestowed on the supposedly conservative New York senator solely because of his opposition to Reagan's fiscal policies. He was picked because, in contrast to some of his supposedly liberal but cautious Democratic colleagues, he has not hesitated in recent months to challenge the administration on any number of fronts.

As vice chairman of the Senate Intelligence Committee, he has been concentrating on an investigation into the tangled personal affairs of William J. Casey, director of the CIA. It has, of course, been acutely embarrassing to the White House, but no more so than some of the seantor's other attacks on the Reagan regime.

He accused the administration of conducting "a campaign of political terrorism" to frighten Congress into slashing Social Security. He opposed efforts to cut housing subsidies and raise rents for low-income tenants. In defending Medicare, the senator said, "In all the talk of these budget cuts, there's almost no attention paid to the most dramatic effect of Medicare. It's changed the lives of old people." He was equally concerned about what would happen to children in foster care if assistance were jeopardized by administration plans to abdicate federal control. The senator thinks the cities, especially New York, are being shortchanged in the Reagan budget, with its reduction of social programs and increases in military spending.

Monynihan has been toughest of all on the administration's foreign policy. He says it doesn't have one -- just "a series of speeches and trips and press statements." He was "appalled at the way we have handled ourselves at the way we have handled ourselves in Asia and Pakistan." He criticized Secretary of State Alexander Haig for offering arms to the Chinese and getting "nothing in return."

Moynihan himself has never appreciated being called a neo-conservative, yet that is the way he has been widely perceived in recent years. The New York Times has offered to him as "a leading apostile of neo-conservative philosophy." In The Washington Post, he was described as "a leading spokesman for a melange of hard-line foreign policies and 'free enterprise liberalism' that has come to be called neo-conservative politics."

As such, Moynihan was closely identified with a prominent group of defecting Democrats and former Democrats who found their party's foreign and military policy too "soft" and its domestic social policies too "extreme." But, unlike Moynihan, many of these old associates are now serving in the Reagan administration or uncritically supporting it.

When Norma Podhoretz, editor of Commentary and a spokesman for the neo-conservatives, first began promoting Moynihan for president, he said, "If I had to invent a candidate to suit the political mood of the country, it would be somebody like Moynihan."

That was in 1978. What would he say today?

The senator used to blast Democratic liberals on the grounds that they believed "government should be powerful and American should be weak." Still, in speaking for the Democrats at the Gridiron dinner this spring, the new Moynihan said, "We believe in American governmnet, and we fully expect that those who now denigrate it, and even despise it, will soon or late find themselves turning to it in necessity, even desperation."

It is hardly surprising, then, that the Democratic leadership is turning to the senator as a liberal spokesman. The Democratic National Committee, in fact, has just launched a fund raising drive with a letter appealing for help in resisting Sen. Jesse Helms (R-N.C.) and other ultra-conservatives who, the committee claims, "now control the Republican Party."

The letter, signed by Moynihan and Rep. Morris Udall, contends that the "mandate" of November has been distorted into a demand, among things, for repealing the Voting Rights Act, outlawing all abortions, subverting Social Security, crippling daycare centers and allowing developers to exploit public lands.

The senator will be up for re-election next year. He also may be a candidate for president in 1984. In either event, he apparently won't be running as a neo-conservative.