President Reagan yesterday praised the sweeping tax-overhaul bill approved by the Senate Finance Committee as "a giant step forward" and "a magnificent first victory over the stagnating forces of the status quo."

He said the bill, if it becomes law, would make an enormous contribution to tax fairness and would remove 6 million working poor from the tax rolls.

Sounding delighted at the turnabout in prospects for a broad-based change in the tax structure, Reagan took the opportunity of his weekly radio address to gibe at "influential lobbyists and insiders" who were saying less than a month ago that tax revision was dead.

"What happened? The people won. That's what happened," Reagan said at Camp David, where he is spending the weekend resting after his 13-day trip to Asia. "America today stands poised to lift off into a new age of opportunity powered by one of the most exciting economic changes of my lifetime."

Reagan said he hopes Republicans and Democrats will unite in passing the measure as soon as possible. He said the bill is "not perfect" but added that it meets the conditions he laid down last December for tax revision.

It is not clear, however, whether the bill is "revenue neutral," neither raising nor lowering the government's income. Reagan has consistently made that one of his conditions.

"As far as I'm concerned, it's a giant step forward," the president said. He said his Council of Economic Advisers regards it as "pro-growth and pro-opportunity" and has estimated that its added incentives and efficiencies could increase the nation's economic growth rate nearly 10 percent over the next decade.

"That could mean as much as $600 to $900 more income per household each year," Reagan said. "Jobs could rise an additional 4 million over the period."

In the Democratic Party's response, Rep. Stan Lundine (N.Y.) said Democrats are prepared to join with the White House "to achieve the most significant tax reform in 30 years" but expect the president in turn to work with them in addressing the deteriorating trade situtation.

During Reagan's administration, Lundine said, "The United States has done too little after awakening too late . . . . The trade deficit has gone from around $30 billion to nearly $150 billion -- a fivefold increase."

Reagan said the tax bill itself would make America more competitive in world markets.

The bill faces determined opposition from some of the nation's most powerful lobbies, including the real estate industry. At a meeting of the Business Council yesterday in Hot Springs, Va., Sen. John W. Warner (R-Va.) said the plan might also run into difficulties because of attempts in Congress to restore defense budget cuts, which he estimated at about $27 billion.

"There will be a tug of war on taxes versus defense," Warner said. He foresaw efforts to restore defense money by means of tax-related measures.

"The strategy to get the tax bill through the Senate," Warner advised, "is to start floor debate on the day the Senate goes on national TV . . . . tell me what senator is going to get up and try to plug a corporate loophole."