Calling American workers "the most important asset" in the U.S. economy, Democratic presidential candidate Sen. Gary Hart (D-Colo.) tonight outlined a national industrial policy that would provide corporations with tax credits to retrain workers and allow individuals to set up tax-deferred savings plans to help finance their own retraining.

Hart, appearing at the second in a series of New York forums for Democratic presidential contenders, said he favored an industrial policy that followed neither the free-market approach of the Reagan administration nor the massive public works programs long favored by his own party.

Instead, he called for "a new system of economic security" for American workers, emphasizing education and retraining.

Hart, 45, struck a variety of themes as he tried to boost his dark-horse candidacy, part of an effort to refocus his campaign on his youth and his desire to provide detailed solutions to the nation's problems. He spoke of his western heritage, but told the audience in this economically depressed city that he would favor federal economic policies tilted to the Northeast.

In answer to a question, Hart said he supported President Reagan's response to the Soviet Union's downing of a South Korean airliner, in which 269 people were killed. But he said he favored continued efforts to negotiate an arms agreement because it was in the national interest.

Hart's industrial policy included a commitment to a 15- to 20-year program to rebuild the nation's roads, bridges and sewer systems. And, in this heavily union town, he devoted much attention to a six-point agenda aimed at helping workers.

The plan includes passage of his American Defense Education Act, an improved national jobs bank, "customized" training for workers through vocational schools, and economic equity for women, which he called his program's moral aspect.

Hart said he favored tax credits of up to 25 percent for firms that retrain their workers rather than lay them off. He proposed Individual Training Accounts, patterned after Individual Retirement Accounts, which would include money from workers, employers and the federal government. The accounts would help employes finance retraining.

Asked by Sen. Daniel Patrick Moynihan (D-N.Y.) whether he favored creation of a new Reconstruction Finance Corp. to underwrite investment costs of rebuilding the nation's economy, Hart said he was not prepared to support something that massive. He said he favored some kind of government loan guarantees for such industries as autos and steel, but said he would not target them to individual companies.

Hart was asked what he would do to restore the economic vitality of a city that has suffered unemployment rates of 8 to 15 percent over the past decade.

"I will not abandon smokestack industries," he said, adding later that he would tell economically healthy regions that they must help pay to repair the battered Northeast.