President Reagan should completely reevaluate the rationale for keeping U.S. Marines in Lebanon because of the escalation of hostilities there, Democratic presidential candidate Reubin Askew said here tonight.

Kicking off a month-long series of forums in New York state for Democratic candidates, the former Florida governor sharply attacked Reagan's economic and foreign policies before an audience of several hundred people in the capital's convention center.

Askew, a dark horse among the six announced Democratic candidates, said that, although there may be a justification for keeping troops in Lebanon, "It's a completely different situation" than when the Marines were sent there, "and it should be reappraised," he said.

Saying he opposes congressional action that would bypass the War Powers Resolution, Askew said Reagan "should be reviewing whether we should keep troops and on what conditions we would keep troops" in Lebanon.

"He should have already started the clock running on the War Powers Act," Askew said.

Askew also criticized the president for "relying too heavily" on a military solution to the conflict in Central America. "There is a communist threat," he said. "It has to be dealt with. But it can be dealt with in a more constructive way."

Askew spent much of the evening discussing economics and attacked Reagan's three-year tax-cut program as "one of the most ill-advised things we've ever done."

If elected, Askew said, he would support repeal of the last installment of the tax cut, a 10 percent reduction that took effect last July 1, would put "on hold" plans to index tax rates for inflation and would "substantially" cut back the rate of increase in defense spending.

"Why don't we admit it didn't work?" Askew said of the administration's economic program, adding that it has not produced additional savings or investment, as Reagan had predicted.

In response to a question from New York Gov. Mario M. Cuomo (D), Askew said he would consider additional consumer taxes and cuts in certain entitlement programs to help eliminate budget deficits approaching $200 billion but only after taking action on basic tax rates.

He said that he favors an aggressive free-trade policy and that the United States should eliminate federal budget deficits to end the imbalance in world currencies.

Askew has staked out positions at odds with many Democrats on abortion and gay rights and was asked about both.

He spoke eloquently on gay rights, acknowledging that he is "struggling" to understand the issue better and saying his thinking has changed as a result of conversations with gays.

Askew said he would not condone homosexuality as "an acceptable life style" and has "a problem" in categorizing this "as another item of civil rights." But he said he would be sensitive about the subject as president, adding, "It's an evolving process with me. I can't guarantee how I'll come down.

He said he believes that life begins "prior to birth" but opposes as too restrictive anti-abortion legislation sponsored by Sen. Jesse Helms (R-N.C.). He favors limiting abortions, he said, and prefers that Congress resolve the issue by statute rather then let courts set the standards.

He said he favors abortions to protect a woman's health in cases involving rape, incest or fetal problems if the woman's health is "defined by statute implementing a constitutional amendment that would put it in the hands of the Congress to deal with it."

Appealing to his New York audience, Askew said he favors channeling more defense contracts to the Northeast and believes that revitalizing the inner cities is a national problem.

In response to questions from members of organized labor, he said that he opposes requiring workers to join unions before they can take a job, as well as so-called domestic-content legislation to aid the auto industry.

Tonight's forum was the first of six regional meetings at which individual candidates are to appear before audiences in different cities. The last is scheduled Oct. 6 in New York City, with all of the candidates attending.

The forums were organized by Cuomo and Sen. Daniel Patrick Moynihan (D-N.Y.), and their existence has kept major political leaders neutral in the Democratic nomination process.