An early spring showdown on the MX missile could be delayed in light of arms control talks with the Soviet Union, House Foreign Affairs Committee Chairman Dante B. Fascell (D-Fla.) said yesterday after Secretary of State George P. Shultz urged congressional support of defense programs as the talks proceed.

While not predicting that the MX vote will be delayed, Fascell said that such a move is "entirely possible."

Asked if the resumption of talks has improved the climate for approval of the missile program, which has been in trouble in both the House and Senate, Fascell said, "I think there's some reason to reach that conjecture."

The talks are expected to have an impact on MX strategy in Congress, although it is not yet clear what, according to congressional leaders. Some believe that wavering lawmakers will be persuaded to vote for the missile on grounds that its defeat could undercut the United States at the bargaining table. Others suggest that uncertainty over the outcome of the vote could force a delay.

Under a complex formula worked out last year to ease passage of critical defense measures, $1.5 billion for 21 new MX missiles cannot be released until after March 1. At that point, if the president requests the funds, two votes of approval by both houses are required before the money can be spent.

To put off the vote, President Reagan could delay the request or Congress could rewrite the legislation to accomplish the same purpose, according to congressional sources, who also suggested that a delay might satisfy both sides if the outcome of the vote is in doubt.

Some key supporters of the MX, fearful of the outcome, have begun talking of a delay, although no formal proposals have been made. However, a long delay could jeopardize target dates for completion of the system, for which only 21 of an anticipated 100 missiles are under production, congressional aides said.

Fascell's comments came after closed-door briefings by Shultz for Senate and House members on the U.S.-Soviet agreement to resume arms control negotiations.

After the briefings, Shultz stressed the need for continued congressional support for the administration's military buildup, including the MX and the president's space-defense program, known as the Strategic Defense Initiative. He said, however, that Congress should support the buildup for security reasons rather than for its influence on the negotiations.

"As we proceed in the process of negotiations," Shultz told reporters, "it is essential that we look continually at what we must do for the security of the United States -- at our defense programs, at pursuing the Strategic Defense Initiative and in many other ways."

Any modification of defense programs should come as a result of negotiations, not before they begin, said the secretary.

"Obviously if they the Soviets can get what they want out of us without giving anything in return, they love that," he said.

Following Shultz to the microphones, Fascell said arguments made by Shultz in the meeting with House members "will be very persuasive," although he added that the interpretation of what is necessary for national security will be open to debate.

After an earlier briefing for senators, Foreign Relations Committee Chairman Richard G. Lugar (R-Ind.) declined to speculate about whether the MX vote might be delayed but suggested a decision on the issue would probably have to be made jointly by leaders of both houses in consultation with the White House and State Department.