The Reagan administration's request for an additional 21 MX missiles cleared its final hurdle in the Senate yesterday but, with action switching to the House, the White House got an unexpected jolt when the House Appropriations Committee narrowly voted against the MX for the first time.

The 28-to-26 Appropriations vote against releasing $1.5 billion for the nuclear missiles came as the generally pro-defense House Armed Services Committee voted 37 to 8 to provide the funds, which Congress froze last year.

House Armed Services Committee Chairman Les Aspin (D-Wis.) said yesterday he believed that Congress had no choice except to vote for the missiles because of the arms talks in Geneva.

"If the negotiations are going on, for Congress to go in and help the Soviets by taking something off the table is goofy," he said.

However, Rep. William V. (Bill) Alexander Jr. (D-Ark.), a longtime MX supporter, switched to vote against the missile yesterday in Appropriations largely because of the budget deficits.

"I support the president's strategic defense modernization policy," he said, "but it is deficient. It has no plan for payment."

The House committee votes yesterday were mostly symbolic. Under a complicated agreement between MX supporters and foes, the decision to release the funds must be made by the full House and Senate.

But lawmakers said the two votes were an indication of the ambivalence many lawmakers feel toward the MX and a sign of how close next week's House votes are likely to be.

The issue spilled over in a caucus of House Democrats late in the day, when opponents of the missile confronted Aspin over his decision to support releasing funds for the MX.

Aspin has been key in helping the administration win approval of the MX in the past, and the opponents charged that he had misled them in his chairmanship race by indicating he would vote against the missile this spring.

Rep. Mickey Leland (D-Tex.) accused Aspin of "tap-dancing" on the issue, but Aspin contended that he had made no commitments to oppose the MX on this vote.

MX foes, who were concerned that the strong votes for the MX in the Republican-controlled Senate had hurt their efforts in the Democratic-led House, said the Appropriations vote yesterday indicated that the fight for continued funding of the 10-warhead nuclear weapon was far from over.

"It shows we have more momentum than MX proponents had thought," said Rep. Les AuCoin (R-Ore.), one of the chief MX foes. "It's a good counterbalance" to the Senate votes, he said.

"The fact that it won in the Senate doesn't mean anything," said Rep. Richard A. Gephardt (D-Mo.), chairman of the Democratic Caucus. "It's a Republican-controlled Senate."

Gephardt predicted that the outcome in the House would be decided by five or six votes.

However, House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.), who opposes the MX, warned that the Senate votes showed how effective White House lobbying is.

"Make no mistake about it, the power and popularity of the president is awesome . . . . It's as awesome as the weapon itself," O'Neill said.

He said that Democratic leadership tallies show House MX votes as "very, very close" and that the leadership, after hesitating to take on the issue, is now "doing everything we can."

The House is scheduled to vote on the MX on Tuesday and Thursday of next week.

O'Neill said that the anti-MX forces have a "solid" block of 190 to 200 votes but that an additional 20 or so lawmakers, who otherwise might have opposed the MX, "are wavering" because of Reagan's arguments that denying him the missile would undermine the efforts of U.S. negotiators at the arms talks in Geneva.

"They all give me the argument for the bargaining chip," he said.

The White House began focusing its lobbying on House members yesterday, with administration officials conceding that the split in the House was closer than many expected.

Reagan met with several Democrats at the White House on Tuesday. Rep. Norman Sisisky (D-Va.), one of those who met with Reagan, yesterday decided to vote against the MX despite the presidential plea.

"When you have a willing buyer and a willing seller and you can't make a sale, then something is wrong," Sisisky said during the Armed Services Committee session. He said he was "not convinced that the Soviet Union went to the negotiating table because of the MX missile."

Reagan made a telephone plea to Rep. Corinne C. (Lindy) Boggs (D-La.) to vote for the missile, but she, too, rejected the president's argument, MX opponents said.

Many Democrats have been especially leery of opposing the MX for fear Reagan will blame them if the arms talks collapse. Democrats are also worried that their party is seen as being weak on defense, so many of them have based their opposition on the weapon's cost, especially in a year when many programs may be cut way back because of the federal budget deficit.

Alexander said he decided to reverse position on the MX when farmers came to Capitol Hill last month asking Congress to help ease their credit crunch.

"The blank-check defense policy" has helped cause the $200 billion-a-year federal budget deficits, he said, adding that there is "a direct connection" between the deficits and the farmers' problems.

Alexander's switch means the House Democratic leadership is united in opposition to the missile.

During the 45-minute debate in Appropriations, Rep. Joseph M. McDade (R-Pa.) defended the MX, saying that a vote against the missile would be "devastating" to the arms talks.

"All of NATO is watching the vote," he said.

But Rep. Bill Green (R-N.Y.) countered that "if you buy that argument you might as well close up the defense subcommittee of Appropriations because we're just going to be rubber-stamping the administration for the next several years" on weapons requests.

The 55-to-45 Senate vote yesterday followed an identical vote Tuesday to authorize continued production. Yesterday's vote completed Senate action on the MX for the current fiscal year.

Although the 10-vote Senate margin was wider than either side had expected, there were continued expressions of ambivalence about the missile system, which could complicate chances for approval of the additional 48 missiles that Reagan wants for next year.

Shortly before yesterday's Senate vote, the Senate Appropriations Committee approved the resolution to release the funds by a vote of only 15 to 14. In passing on the fiscal 1986 defense appropriations bill, that committee will play a critical role in determining the fate of the additional 48 weapons.

Yesterday's three-hour debate on the Senate floor was desultory and anticlimactic, dominated by Democratic MX foes who continued to argue against the weapon despite the foregone conclusion of the vote.

Their frustration was voiced by Sen. Dale Bumpers (D-Ark.), who complained that it is virtually impossible to kill a weapons system once it has been started.

"Weapons systems have gotten to be just like Rasputin -- you can't kill them," Bumpers said.

"It doesn't make any difference whether they work or not," he added, citing the Pentagon's continued funding requests for the Divad antiaircraft gun despite the fact that it won't fire properly in cold weather.

Republican supporters of the MX expressed a different kind of frustration, contending that the Senate shouldn't be forced continually to spend time on the MX when it has voted for the system repeatedly and pumped billions of dollars into its development.

The Senate has voted on the MX 29 times in the past 11 years, said Sen. Ted Stevens (R-Alaska), chairman of the Appropriations subcommittee on defense. "I don't know why we have to vote again and again on a system we've already spent $13 billion on," Stevens added.