Washington — Democratic presidential candidate Walter F. Mondale, at the beginning of a new and more critical point in his presidential nomination campaign, charged that ''a dangerous escalation of the arms race is under way'' under Ronald Reagan. Mr. Mondale, speaking before the National Press Club Tuesday, promised to hold an immediate summit conference with the Soviets, if elected, and to institute regular summit conferences thereafter.
Speaking in calm, level terms, the former vice-president delivered one of the sharpest and most comprehensive attacks yet made against the President, particularly stressing the nuclear peril. He was applauded when he repeated a plea ''for the withdrawal of our Marine forces at Beirut'' and again when he pledged, if elected, to resume the failed SALT II talks for nuclear disarmament.
Mondale insisted that he would keep American defenses strong. He assailed a situation where now, he insisted, the superpowers are ''further from an arms agreement than at any time that I can remember.'' He hailed the Rev. Jesse Jackson for securing the release of Lt. Robert Goodman in Syria.
It is a decisive time for the former vice-president because he seeks to dispel his misty national image and become a clear political reality. In opinion polls he is well ahead of other Democratic hopefuls. Among the seven who trail him, Sen. John Glenn of Ohio is runner-up, and the Rev. Jesse Jackson has seized national headlines by arranging the release of captured US airman Robert Goodman Jr. from Syria. The time has come, Mr. Mondale's camp has decided, for him to take a more active role.
Mondale's more active role began last week when he announced that he favors the early withdrawal of US troops from Lebanon. It was the most positive statement he had made and coincided with conferences here of party leaders called by House Speaker Thomas P. O'Neill Jr., formerly a supporter of the White House operation in Lebanon, but now veering to opposition.
Events are turning campaign attention to foreign affairs to an unusual degree. They also affect Mondale's competition with Mr. Jackson. Mondale had questioned the Jackson mission to Syria and has actively competed with Jackson for black votes.
Another new feature is the low-keyed but basically powerful condemnation of the Reagan intervention in Lebanon made by the so-called Long commission (named after its chairman, retired Adm. Robert L. J. Long), following the Beirut truck-bomb tragedy in which 241 marines were killed. President Reagan has absolved many in the military chain of command from responsibility for the disaster, assuming blame for himself, but this opens the way for Democratic attacks. The commission infers that the whole policy should be reexamined and altered.
Washington is waiting to see what use Mondale makes of this accumulating material and is ready to appraise him anew as a presidential contender.
The first state test vote is tentatively set in Iowa, Feb. 20, and in New Hampshire, Feb. 28. The Mondale campaign has been better organized and better financed so far than its rivals. But can he make his image real to millions of only mildly interested voters, and use to his advantage the political ammunition now building?
Besides his appearance here, Mondale has scheduled a five-state trip to the South (in Alabama, Arkansas, Georgia, Florida, and Mississippi) where he is challenged by Democratic presidential rivals.
These will test out how Jackson's standing may have changed, following his spectacular headline-grabbing action in getting Lieutenant Goodman released, and the effects of Mondale's own reaction to the Lebanon developments.
The Long commission, for example, declared, ''The commission therefore concludes that there is an urgent need of alternative means to achieve US objectives in Lebanon and at the same time reduce the risk to the Marines.'' The commission says flatly that ''the (Marines were) not trained, organized, staffed , or supported to deal effectively with the terrorist threat.''