To soften the plastic modernity of the brand-new Hyatt Regency Hotel in Baltimore's Inner Harbor development, Democratic Party aides hung posters of past campaigns and quotations from past Democratic presidents. The idea was to make the visiting Democratic officials feel as if they were in comfortable, familiar quarters.

But in fact there had been no such between-conventions gathering for the Democrats since the late 1950s. And when the Democratic National Strategy Council convened its first meeting on Friday night, no one was certain that it might not be the last.

The weekend meeting of federal, state and local elected officials represented the biggest gamble the Democrats have taken in the policy area since the bitter confrontations over Vietnam and the emerging social issues split the party in the mid-1960s and ushered in what most Democrats see as a 15-year period of decline.

But by the time the 50-some Democrats reached their windup session late Saturday afternoon, it was clear that the strategy council would survive, if not necessarily succeed.

That closed-door session produced general agreement on two key points: The Democrats need clear national themes to prevail in the 1982 election. And the strategy council itself needs more closed sessions if it is to come up with the themes.

Pollster Patrick J. Caddell made the argument that "1982 will be a party election like we've never had before," and therefore "Democrats need to struggle together for some common program." The public judgment on the Reagan program is likely to dominate not just the congressional campaign but state and local elections as well, Caddell said, and the Democrats will be bowled over individually by the orchestrated and well-financed White House-Republican effort unless they have common themes of their own with which to compete.

Rep. Gillis W. Long of Louisiana, the chairman of the House Democratic caucus, made the plea for more closed sessions of the council, saying that it was only outside the sight of the cameras and the hearing of reporters that elected officials could drop their posturing for their own constituencies and engage in the serious search for agreements. Only in the last 90 minutes of last weekend's session were the Democrats unchaperoned by the press.

Both messages were greeted with agreement, Democratic officials said yesterday, and the upshot is that task forces are likely to be meeting -- without press scrutiny -- before the end of this year to start the search for what may ultimately become strategy council and party positions.

The first two areas will likely be the national economy and federal-state-local relations, with problems of foreign policy and defense ticketed for later consideration. Formal ratification could take place, if all goes according to plan, at next June's Democratic mini-convention in Philadelphia.

That Democrats would even contemplate launching such a process seemed unlikely a few months ago. The first aide selected by party chairman Charles T. Manatt to sound out congressional attitudes on the project ran into such serious problems that he went home to California in frustration.

Senate Minority Leader Robert C. Byrd (D-W.Va.) was particularly obstinate, fearful that the strategy council would usurp or preempt congressional policy-making prerogatives. To assuage him, the word "policy" was dropped from the council's title and the question of whether it would ever issue policy statements in the party's name was left hazy.

The concessions were enough to get Byrd and House Speaker Thomas P. (Tip) O'Neill, Jr., (D-Mass.) to come to Baltimore Friday to give the council their blessing. Other big-shots also showed up briefly -- Sen. Edward M. Kennedy (D-Mass.), former vice president Walter F. Mondale, ex-party chairman Robert S. Strauss.

None of them hung around long, which did not disappoint the organizers. While they wanted the legitimacy the big names could confer, they did not want the council to become an arena for 1984 presidential politics, and they did want to focus national attention on the younger-generation Democrats who represent the party's long-term future.

More than half those invited came to the session, and they plainly enjoyed getting to know each other, many for the first time. The willingness of such well-known figures as California Gov. Edmund G. (Jerry) Brown Jr. and House Budget Committee Chairman James R. Jones to spend a weekend at the conference table impressed the newcomers to the national scene. And the comments of such newcomers as Vermont Lt. Gov. Madeleine Kunin, St. Louis Mayor Vince Schoemehl, Oklahoma State Rep. Cleta Deatherage, Travis County (Tex.) Commissioner Ann Richards and Essex County (N.J.) Executive Peter Shapiro impressed many of the members of Congress.

"There are clearly strong disagreements among us on many issues," Rep. Michael D. Barnes of Maryland said, "but there is a feeling we can -- and must -- get our act together."

That Democrats are agreed at least on that proposition was probably most candidly explained by Rep. Charles B. Rangel of New York. "Our common thread today," he said Saturday, "is that we're losers. In the past, we've never had to get along; we've been able to get by just tolerating each other. Now we've reached the point we have to find out what we do agree on. And we have to find out how to handle the issues where we don't agree."

That the Democrats have grasped that basic point may be the key to survival, if not success, for more than the strategy council.