Former defense secretary Harold Brown yesterday strongly endorsed construction of the MX intercontinental missile but said the Reagan administration's "Dense Pack" deployment system "does not solve the vulnerability problem."

Brown's comments, on "This Week With David Brinkley," (ABC, WJLA) marked the first time the Carter administration defense chief has spoken out on MX since the Reagan administration announced plans to base 100 MXs in concrete- and steel-hardened silos placed closely together at an Air Force base near Cheyenne, Wyo.

Brown, a nuclear physicist, was a key military strategist in the Carter and Kennedy administrations, and his opinions are expected to have a considerable impact among Democrats in the congressional debate on MX.

As defense secretary, Brown advocated building 200 MX missiles and moving them among 4,600 shelters so the Soviets would have to attack all such shelters.

He said yesterday that a land-based intercontinental missile system is necessary for "military and international political reasons" but that "the Dense Pack basing system does not solve the vulnerability problem."

Sen. Henry M. Jackson (Wash.), second-ranking Democrat on the Armed Services Committee, said on "Meet the Press" (NBC, WRC) that the Dense Pack basing proposal is in "deep, deep trouble" in Congress.

Defense Secretary Caspar W. Weinberger and Sen. Ernest F. Hollings (D-S.C.), a leading MX foe, also appeared on the Brinkley program and gave a preview of congressional battlelines in the MX debate.

Hollings called the basing plan "a wish in the dark" and said opponents of the proposal have enough votes to defeat it. He said anyone who had proposed putting "all the eggs in one basket" a decade ago would have been a candidate for a "straitjacket."

He said existing Minuteman missiles, now scattered across the West, could be placed in a dense pack configuration if such a system actually works. He said he doubts that it would.

Weinberger argued that MX is needed as a bargaining chip at strategic arms reduction talks in Geneva. He said it is necessary to maintain a triad of nuclear missile systems -- launched from land, airborne bombers and submarines -- not only for defense but also as a lever for arms control negotiations. He said the Soviets are moving toward countering the triad's air and sea legs.

Georgi Arbatov, director of the Soviet Institute of the United States and Canada, appearing separately on the same program, reiterated the Soviet charge earlier last week that attempting to use MX missiles as a bargaining chip is "blackmail" that would aggravate the arms race.