Washington — Sen. Edward M. Kennedy's decision not to run for the White House in 1984 radically alters the nomination outlook for both parties.
For the Democrats, Mr. Kennedy's withdrawal:
* Opens the gate for as many as 10 or 12 Democratic candidates, across the party's spectrum. ''Wide open,'' the Democratic nomination is now labeled.
* Enhances the lead position of former Vice-President Walter Mondale, and pushes forward Sen. John Glenn (D) of Ohio as the likely challenger to Mr. Mondale.
* Reduces the prospect of a bitter convention fight to dislodge Kennedy, if a convention showdown loomed and party pros wanted to turn to a more centrist or conservative candidate as the best prospect to defeat the Republicans.
Kennedy ruled out accepting a draft for president or vice-president at the party convention, but hinted broadly at a possible 1988 presidential run.
For the Republicans, Kennedy's early withdrawal:
* Deprives Reaganites of their favored major opponent for 1984. Kennedy was seen as drawing the sharpest ideological and historical contrast with President Reagan. Kennedy's past personal problems and his politically liberal reputation could have handed most of the South to Reagan - the region clearly most crucial in the next election. Mr. Glenn, at the moment, is the most feared Democratic prospect, with his folksy, national-hero aura - though he remains untested in the rigors of serious presidential cam-paigning.
* Gives Reagan reason to reconsider carrying the Republican banner in 1984. Kennedy's absence lessens a clear sense of ideological challenge. Among GOP White House would-bes: Congressmen Jack Kemp of New York and Philip M. Crane of Illinois on the far right, Vice-President George Bush in the center, and moderate Sens. Howard Baker of Tennessee and Bob Dole of Kansas.
For Kennedy himself, those close to him say his chief reason for dropping out - family opposition - should be accepted at face value. ''The kids were dead set against it,'' said one close to the decision process. ''It's understandable, given what the long haul of a campaign would mean.''
The political realities for Democrats generally did not make a run for Kennedy any easier. Many pros think that, whether Reagan runs or not, the odds are still against a Democrat's winning in 1984.
Kennedy aides told the Monitor that the senator had a chance to win the White House, not just the nomination. ''If the economy is poor next year, if the public is yearning for strong leadership - then it's a different environment,'' one aide said. ''They'll want someone to turn things around.
''If there isn't fear and anger out there, it'll be difficult for any Democrat. Nobody knows what the landscape will be. Also, once Kennedy got the nomination, it would have been a different situation for him.''
Kennedy's bowing out leaves the Democrats with two broad alternative types of candidates for 1984. The pragmatic centrists - Mr. Glenn and Mr. Mondale - will not have the field all to themselves. A second group of idealist candidates is likely to emerge, following the vacuum left by Kennedy, the party's most charismatic leader.
The last two elections have been dominated by middle-aged and senior populace concerns. Youth - their excitement, interest, participation - was the most notably missing element in the last two elections. Republican John Anderson may try again to build an independent candidacy around those segments of the electorate unaroused by political appeals.
''The fact the chief liberal is out does not mean it will not be an ideological campaign,'' says one Democratic strategist, who sees young voters as a potent latent force in the next White House round.
''There is still a strong longing within the Democratic Party for some clear alternative to the current Republican themes. They want to take a stand this time. Some candidate could catch this feeling. The younger generation is not as liberal on economic issues as their parents were, but they still could rally around somebody who is a compelling, visible figure.''
In announcing he would not run, Kennedy emphasized the Democrats' basic party strategy for 1984 - challenging Ronald Reagan more on Capitol Hill and nationally, calling for big changes in his program to underscore a GOP-Democratic contrast.
The most likely economic scenario calls for mixed results as the election approaches - lingering unemployment, but greater profitability for businesses and working individuals who benefit from the economic recovery. It could frustrate the Democrats' efforts to draw a clear contrast with the Reagan Republicans.
In practical political terms, Kennedy's withdrawal leaves a large activist wing of his party - the individuals who work in campaigns, run as delegates to conventions, wage the intraparty debates over platform planks - free to seek new allegiances. This, plus new convention rules which will seat a large group of delegates at the convention uncommitted, multiplies the wide open picture left by Kennedy's departure.