Former vice president Walter F. Mondale and Sen. Edward M. Kennedy (D-Mass.) took President Reagan's economic program to the woodshed yesterday, where they swung away with metaphors and one-liners and tried to flatten it with their own economic proposals.

Sen. Lowell Weicker (R-Conn.) joined the Democrats' assault before 4,000 delegates to the annual legislative conference of the AFL-CIO's Building and Construction Trades Department.

The delegates seemed to enjoy the show, giving the liberal trio frequent rounds of applause. One day earlier, the delegates had given President Reagan a chilly welcome sprinkled with boos.

Both Kennedy and Mondale, potential rivals for the 1984 Democratic presidential nomination, urged Reagan to abandon his multi-year-tax-cut strategy and to pressure the Federal Reserve Board into reducing interest rates.

Both said Congress should repeal the personal tax cut for 1983, eliminate the new tax-leasing provision and further restrict the application of other corporate tax cuts.

Failure to eliminate the 1983 individual tax cut could lead to a "truly staggering . . . $200 billion" federal deficit, Kennedy said.

Reagan consistently has rejected suggestions that he abandon his tax plan, a point Mondale noted yesterday.

"Everyone is pleading with the president to abandon his program. . . . But the president won't budge," Mondale said. "He's seen too many westerns, the ones where the Lone Ranger toughs it out on his own until the cavalry comes to his rescue.

"What the president doesn't realize is that it's already high noon for our economy," Mondale continued. "The hands at the O-K Corral have all been laid off. Tonto's unemployment benefits have just run out."

To alleviate unemployment, Mondale and Kennedy said the administration should begin job-training, public works and public-housing construction programs. The administration has committed itself to only one of those proposed remedies--a $1.8 billion job training program proposed for fiscal 1983.

"We have all heard the administration's plea to wait a little longer" for the economy to improve, Kennedy said. He said the president's success in buying time with that plea "deserves an Academy Award."

Kennedy accused Federal Reserve Chairman Paul Volcker of doing the "administration's dirty work" by following a "strategy to savage the housing industry in order to salvage the Reagan tax cut." The senator said the Federal Reserve should be stripped of its independent status and "put . . . in the Treasury Department, where it belongs." Credit controls channeling loan funds to "high-priority areas" also should be imposed to help reduce interest rates, Kennedy said.