With the skyline of Balitmore providing the backdrop, a construction worker, wearing a blue hard hat and with a toothpick protruding from his mouth, looks into the camera while an unseen voice questions him.

"What do you know about Maryland's liberal senator, Paul Sarbanes?" the voice asks.

The construction worker shrugs his shoulders, indicating not much.

"Maybe he wants it that way," the voice suggests ominously, going on to describe Sarbanes as "the biggest spender in the Senate," a man who "voted to increase federal spending by a staggering $231 billion," an opponent of both tax cuts and President Reagan's economic program. The message of the 30-second television commercial is that the construction worker clearly has heard enough about Sarbanes to vote against the first-term Democrat when he comes up for reelection next year.

Beginning Wednesday, the hard hat and the hard anti-Sarbanes message will be shown on Wednesday and Balitmore television stations, part of a $1 million campaign to sell the Reagan administration's economic program and advance the cause of a conservative poltical action committee that specializes in targeting liberal members of Congress for defeat.

The campaign was announced yesterday by John T. Dolan, chairman of the National Conservative Political Action Committee, an organization that claims credit for defeating a number of liberal Democrats in 1980 and which has already tentatively targeted 20 liberal senators -- including Sarbanes -- for defeat in 1982.

Dolan also announced that for the first time NCPAC has targeted House members for defeat, beginning with what he called the three "most obstructionist" House Democrats standing in the way of enactment of the Reagan economic program -- Majority Leader Jim Wright (D-Tex.), Budget Committee Chairman James R. Jones (D-Okla.) and Ways and Means Committee Chairman Dan Rostenkowski (D-Ill.).

Dolan conceded that, the targeted congressmen "are going to be very difficult to defeat," particularly Rostenkowski and Wright, but he added that "they do not represent their districts" and therefore should be vulnerable.

NCPAC's advocacy of the Reagan economic program, which Dolan criticized yesterday for not going far enough in cutting spending, is a departure in tactics for the conservative organization, which in the past has specialized in negative advertising directed at liberal Senate Democrats. Linking its attacks on the four Democrats to the president's program, however, is likely to improve fundraising prospects for the organization. Dolan said NCPAC has already raised $450,000 for the effort, adding that he sees "no problem in exceeding the $1 million goal."

Sarbanes is NCPAC's first target this year not only because of his "big spending" voting record but because his state is covered by the Washington media market.

Dolan hopes that other members of Congress, seeing the commercials on television in their Washington-area homes, "will substitute their own names" for that of Sarbanes and consider what such a campaign might do to them.

Dolan said NCPAC plans to spend about $400,000 trying to defeat Sarbanes, $450,000 against Wright, $100,000 against Jones and $50,000 against Rostenkowski. He said because of the high cost of media in Chicago, the anit-Rostenkowski campaign will be confined to telephone banks and a direct mail effort.

NCPAC's campaign is one of the most ambitious efforts revolving around the president's economic program, but not the only one. The Chamber of Commerce yesterday released a poll it commissioned showing broad public support for the Reagan program and almost unanimous agreement that the economy is in bad shape.

The Republican National Committee is producing two special publications that provide Republican officials across the country with arguments and data supporting the Reagan plan. The RNC is also discussing the possibility of producing its own television commercials supporting the economic package in conjunction with the GOP Senate and House campaign committees, officials said.