When House Democrats met in closed caucus this week to discuss the forthcoming vote on President Reagan's request for 21 additional MX missiles, the session turned into a debate over not the missile, but a man.

He is House Armed Services Committee Chairman Les Aspin (D-Wis.), who was elected chairman just three months ago in an effort to give the Democrats a stronger voice and clearer image on defense matters. But, as the 99th House approaches its first major defense vote, Aspin is backing Reagan against the entire Democratic leadership and a great majority of House Democrats.

At the two-hour caucus, angry MX opponents, whose votes helped make Aspin committee chairman, charged that during the chairmanship race he misled them into believing that he would oppose the MX. But these accusations were also part of the broader frustration over Aspin's pivotal role on the issue.

By "playing ball with the White House" on the MX, said one Democrat, Aspin has "blurred" the lines between the two parties on the first major issue of the year.

Democratic leaders caucused this week in large part to show newer members, or those undecided and subject to intense White House pressure, that Aspin, for all his expertise, does not speak for a majority of Democrats on this issue. He is on a "solo flight," said MX opponent Les AuCoin (D-Ore.), that is hurting the party.

"We called the caucus to show those members who are most mesmerized by the Aspin mystique just how infuriated most caucus members are" by Aspin's willingness to carry the administration's fight inside the House, AuCoin said.

Both sides in the battle recognize that the shrewd, persuasive Aspin has the power to help determine the outcome in the votes next week.

"Without [Aspin], we lose. It's that simple," conceded one administration official.

Said a Democratic aide: "He's perceived as a person who's skeptical of the Pentagon and has always been. It would be as if Sam Stratton came out against the MX." Stratton is a strongly pro-defense New York Democrat.

An acknowledged defense expert, who has not supported many administration weapons proposals, Aspin has always exercised strong influence on a substantial bloc of moderates in the Democrat-controlled House.

In his new position as Armed Services chairman, he provides even more "cover," as one lawmaker put it, for many Democrats who believe that the party is seen as "soft" on defense and are worried about bucking Reagan on an issue the president deems vital to arms negotiations with the Soviet Union.

As one of those Democrats, Rep. Vic Fazio (Calif.), who said he has decided to support the MX, put it: "This is one of the most symbolic votes I've ever seen . . . . you're voting on Armageddon or you're voting for appeasement."

Aspin's sway with this group has become especially important in administration calculations this year, White House officials said, because virtually all of the other House Democratic leaders associated with the moderate bloc have moved to oppose the MX in the last two years.

Majority Leader James C. Wright Jr. (Tex.), Majority Whip Thomas S. Foley (Wash.) and Democratic Caucus Chairman Richard A. Gephardt (Mo.) all have become MX opponents. And just this week the chief deputy whip, Rep. William V. (Bill) Alexander (Ark.), joined them, voting in the Appropriations Committee against the administration's request to release $1.5 billion frozen by Congress last year for the additional 21 missiles.

Aspin's support for the controversial nuclear weapon is not new. Two years ago, when it appeared that the House might kill the MX, Aspin led a group of moderates and worked out an agreement with Reagan to save the MX by trading votes for the missile in return for administration progress on arms control.

Since being tied to the arms control process, the MX has survived a series of votes in Congress. Many Democratic opponents of the MX have never forgiven Aspin, saying that once the missile became a factor in arms control it could never be defeated. The Wisconsin lawmaker narrowly avoided being censured for his MX role by home-state Democrats at their 1984 convention.

Many liberal Democrats, once closely allied with Aspin, now refer to him as "a former liberal," and they are the most irritated with him today, particularly over what they say he promised them during his campaign to unseat the aging incumbent Armed Services chairman, Rep. Melvin Price (D-Ill.).

Younger House members led the fight to replace Price, complaining that he had lost control of the committee and allowed it to operate as a rubber stamp for the Pentagon. Aspin was viewed as younger, more aggressive, and more likely to be tough with the administration, qualities that Democrats believe they need to stem GOP gains in the House and to prevent another presidential election loss.

The vote to oust Price, over the objections of the Democratic leadership, was very close. Several liberals asked Aspin for assurances that he would oppose the MX before they would definitely support him.

"He told me if I left the details to him, he would find a way to oppose further production of the MX missile . . . this year," said Rep. Edward J. Markey (D-Mass.), an MX opponent.

Aspin since has repeatedly denied making any such promises. "I told them . . . there were no commitments," he said, adding that he believes that the MX should be available to be bargained away in the arms talks.

One Aspin associate, who helped in the coup against Price and is anti-MX, said, "I don't think he promised anything [but] the problem is that he chose his words carefully . . . and he knew [liberals] were misinterpreting his words. He knew what he was doing and no one else did."

Several lawmakers said this week that the controversy has "definitely tarnished" Aspin, though it is unclear whether that will hamper his effectiveness in the two MX votes next week.

Aspin plainly has been uncomfortable with the controversy.

Last month he met with House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) and suggested that the vote on the request for the 21 additional missiles be postponed and considered in a package with the 1986 request for 48 missiles. That could have allowed Aspin to keep the program alive for the arms talks but to support steep cuts or a cap on the MX program over the two years and thus placate MX opponents.

Aspin, at O'Neill's urging, approached the White House about the delay but was told no, firmly. House Democratic leaders said they believe that the White House might have agreed to a delay if they thought Aspin would oppose the MX. An administration official said, however, that the White House did not know how Aspin would vote when the no-delay decision was made.

Aspin this week blamed MX opponents for the House dilemma, accusing them of "shooting themselves in the foot" by rejecting his 1984 proposal to freeze funding for the weapon unless Reagan reported that the Soviets would not return to arms talks or were refusing to bargain in good faith.

Instead, the MX opponents, arguing that Aspin's proposal gave Reagan too much discretion, won approval of a freeze that allowed the money to be released only if Congress voted twice in each house to do so.

Last week, the Senate voted twice to release the funds, and now House Democrats voice fear that the same thing will happen there next week.

MX opponents say that Aspin, if he were true to the spirit of his earlier proposal, would now vote against the administration's request to have the funds released because the Soviets are back at the negotiating table.

Aspin responds that such a vote would give the appearance of undercutting the administration at the Geneva arms talks. He voices little sympathy for the position MX opponents now find themselves in.

"Basically, I'm telling them it's their own damn fault," he said.