Last Wednesday was a historic day. The television spectacular was the magnificent royal wedding, but American history books will probably record as more significant that this was the day that almost 50 years of Democratic-dominated economic and social policy came ot an end. The budget and tax victories won by President Reagan on both sides of the Capitol reversed the policies Congress had followed under every president from Franklin D. Roosevelt to Jimmy Carter.

By coincidence, at the same time that the House of Representatives was preparing to give Reagan the tax-bill victory that sealed his domination of that nominally Democratic body, a panel at the National Conference of State Legislatures here was discussing the question of the role of party discipline in legislative bodies.

The only onclusion a person could reach from that panel is that the conservative policy and political power so spectacularly displayed in Washington is no surface phenomenon, but a pattern that extends across the country and into the reaches of state government as well.

There were four speakers on the panel. The three Democrats all said -- in plain and sometimes almost cynical terms -- that if you wanted to invoke party loyalty in defense of Democratic goals, forget it. The lone Republican, William Polk, the speaker of the state of Washington house of representatives, was also the lone exponent of the view that party discipline can be used to achieve party objectives.

Despite the fact that Washington state has no party registration for voters and a "blanket primary" that encourages ticket-splitting, Polk said, "the Republican Party has become a very important force, at least among members of the legislature." It's become an election machine," he said, and, largely as a result of the funds and services the national and state GOB provides legislative candidates, "we have a strong caucus."

The same is true at the national level. By building a strong party, with strong leadership at both ends of Pennsylvania Avenue, Republicans have been able to harness the tools of modern politics to the service of thier common goals.

The testimony of the three ydemocrats was dramatically opposite. State Rep. George Fettinger of New Mexico explained why he and 10 other conservative Democrats had broken party ranks in 1979 and again this year to form a conservative coalition with 26 Republicans that elected a renegade Democrat as speaker, against the Democratic caucus choice. "The Democratic Party in New Mexico," Fettinger said, "is completely out of touch with the electorate. No Democrat can run for office in my part of the state on its platform -- and still be elected."

Assemblyman Willie Brwon then detailed how he had been elected as the first coalition speaker of the California assembly last winter, by exploiting a split in the Democratic caucus and cutting a deal in which Republicans supplied 28 of the 51 votes that he received. Brown is a black legislator who describes himself as "one of the most liberal Democrats in existence." He said he had no trouble dealing with "20 to 25 Republicans who come from districts where white sheets are regarded as formal attire. . . They made certain requests. They were relatively modest. They wanted everything except the speakership."

He had no trouble accommodating them, he said, and his GOP friends had no trouble his liberal positions. The reason is simple: "Seldom wil you find me using the office of speaker to enhance the position of the Democratic Party."

Richard M. Scammon, the influential election analyst and political commentator, was the third self-identified Democrate on the panel. He explained that, in his view, "parties are big empty shells. You can fill them up with anything. The basic policy of any party is mush, mush and more mush."

Scammon said that Ronald Reagan had ben elected in 1980 largely on his personality, not his policies, and added that, because of the "flexibility" of political parties, it is almost impossible to maintain discipline in a party caucus in Congress."

Scammon did not explain how, even as he spoke, Reagan and the Republicans were about to reverse 50 years of budget policy and put through the biggest tax cut in history by maintaining almost unbroken party discipline in the House and Senate.

My own reaction to the discussion here and the news from Congress is that the so-called disarray of the Democratic Party goes far deeper than most of its nominal leaders in Washington will admit. At every level of government, Democrats as liberal as Brown, as conservative as Fettinger and as smart as Scammon have become adept at inventing political and intellectual rationalizations for rejecting the cloak of party loyalty.

At the same time, they have managed to obliterate from their consciousness the recognition that Republicans have rediscovered the untility of a political party as a means not just for gaining office, but for remaking policy -- in very large dimensions.

That is an epochal development -- and one that is likely to shape, not just this year, but this era of American politics.