This is likely to be the week the Democrats lose another of their comforting illusions: the belief that everything would be okay, if only the Old Guard would stand down.

Long before Walter Mondale lost 49 states and the intraparty critics began saying "never again a New Deal liberal," there was a rising chorus of Democrats crying: "Give the new folks a chance."

Well, this is the week that the new-breed Democrats step front and center in the national debate on a major issue -- the continuation or extinction of the MX missile program. The country is hearing a great deal from Rep. Les Aspin (D-Wis.), the newly elected chairman of the House Armed Services Committee, and Rep. Richard Gephardt (D-Mo.), the newly elected chairman of the House Democratic Caucus.

Aspin, 46, with degrees from Yale, Oxford and MIT, and Gephardt, 44, with degrees from Northwestern and the University of Michigan, are clearly two of the Democrats' best and brightest. After relatively brief House service -- 14 years for Aspin and eight for Gephardt -- they have been recognized and elevated by their peers over more senior and experienced rivals.

This week, as the House debate on the MX moves onto television and the front pages, the whole nation will have a chance to see their sterling qualities.

It would be a great tonic for the down-and- out Democratic Party, except for one small thing. Aspin and Gephardt are on opposite sides of the fight, and are doing their best to defeat each other.

Gephardt, as caucus chairman, is doing his utmost to rally undecided Democrats to the majority position of the party, as expressed in the House, the Senate and the last national convention. That position is that the MX missile is a dangerous first-strike weapon, housed in vulnerable silos, and fit neither as a bargaining chip nor as a credible piece of military machinery.

Aspin, on the other hand, is trying to hold together a large enough minority bloc of Democrats to give President Reagan the weapon he wants. He says the MX may be only "marginally needed" or marginally useful as a weapon, but "I don't see how Congress can vote against those 21 missiles without weakening the bargaining position" of American negotiators dealing with the Soviets on arms control in Geneva.

Whichever view prevails in the House -- and the betting is that it will be Aspin's -- the Democrats lose. If the House repeats the Senate pattern, where a minority of Democrats voted with most of the Republicans to give Reagan a victory, then the Democrats will have the worst of both worlds. They will, once again, have identified most of their officeholders -- and the party in general -- as being anti-MX, confirming what some Democrats believe to be a public impression that they are "soft on defense." At the same time, the Democratic dissenters will furnish enough votes to confirm the prevailing belief that Reagan is a powerful leader who gets whatever he wants.

In short, in the first big legislative battle of the post-Mondale era, the first where the generals are new-generation Democrats, the party will have once again proved itself both squishy-soft and ineffectual.

So what's new? What's new is the cast of characters. Back in the bad old days of 1968, 1972, 1976, 1980 and 1984, when one Democratic presidential hopeful seemed to be taking the shillelagh to another's head, there was always that murmuring about "old men fighting yesterday's battles."

Well, these are new men, fighting today's battle, and the picture is not much different.

When I asked Gephardt in an interview last week whether he felt any embarrassment at the about-to-be-advertised split in Democratic ranks, and about the role his contemporary, Aspin, was taking in the intraparty ght, this is what he said:

"We can use the caucus to help people understand the merits of the arguments, but we can't use it to impose discipline. It's an effective device for achieving as much consensus as we can, but we're never going to have conformity among Democrats."

Gephardt also said, "I respect Les's position. He has to do what he believes is right."

If I had closed my eyes as he spoke, bless me if I wouldn't have thought it was Speaker Tip O'Neill, or former speakers Carl Albert or John W. McCormack -- or one of those other "old men" the young bloods said just lacked the courage to tell a powerful committee chairman that he owed some loyalty to the Democratic Party and the position the majority of his colleagues had endorsed.

But, of course, these are no fuddy-duddies, but the very model of the modern generation, super-bright Democrats. This is the week they discover that there is more to successful politics than proclaiming themselves the new breed.

New breed, they are. But there's still the old need. It's the need to find enough followers to allow anyone to lead. Without that, they won't be a party. They will remain a rabble.