The Democratic Party's new National Strategy Council launched itself on a tidal wave of rhetoric tonight, with congressional leaders proclaiming that Democrats have found the key to unity in opposition to President Reagan's economic program.

"Ronald Reagan has brought the Democrats together," House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) proclaimed at the kickoff news conference of the party's new policy body.

Saying that spring and summer talk of "Democratic disarray" is now "passe," Senate Minority Leader Robert C. Byrd (D-W.Va.) said, "If you want to see disarray, look at the Republicans. They would rather go out in a T-shirt in a hailstorm than bring a tax or appropriation bill before the Senate, because of their own disagreements."

The "strategy council" is supposed to serve as a policy-making voice for the Democratic party. But Democratic National Chairman Charles T. Manatt, who went through prolonged negotiations with nervous congressional leaders about its makeup and powers, announced that the initial meeting, which runs through Saturday, would be organizational rather than substantive.

The first news conference was an occasion for upbeat rhetoric, with O'Neill, the principal fall-guy in Reagan's early blitz of Congress, enjoying the opportunity to turn the tables. Recalling that he had expressed fears six months ago that 1982 might be the election that signaled a Republican realignment, the speaker said he now expects it to be a "great Democratic year." Both Reagan's economic program and the national economy are "collapsing," he said.

Sen. Edward M. Kennedy (D-Mass.) picked up O'Neill's theme and argued that the Reagan program has now been revealed as one which offers "tax cuts for the rich, Social Security cuts for the elderly and school-lunch cuts for the young."

Sen. Paul S. Sarbanes and Rep. Barbara A. Mikulski, the two Maryland Democrats who welcomed the conference to the newly opened Hyatt Regency Hotel in the Inner Harbor, said that Baltimore's revitalization was made possible by Democratic programs and Democratic political leadership