Moments before President Reagan went on television Thursday night to report to the nation on the summit, the six Democrats vying to succeed him were at a candidates' forum in New York being asked about arms control.

One jabbed at the president for not having accomplished more sooner. Another attacked the right wing of Republican Party for trying to block the Intermediate-Range Nuclear Forces (INF) Treaty signed Tuesday by Reagan and Soviet leader Mikhail Gorbachev. Another called for the United States to "seize the initiative" in the next round of talks with the Soviet Union. Another proposed that the destruction of the missiles be televised.

The trouble with the answers, one Democratic strategist later lamented, is that they all appeared to be "carping around the edges."

None of candidates was ready to confront head-on what could be a double-barreled foreign policy appeal for the 1988 campaign: It took Reagan's tough talk and military buildup to bring the Soviets to the table; it will take an equally vigilant Republican successor to push arms reduction forward.

Republicans are not accustomed to holding the high ground in the debate over arms control, and in the months ahead, they will have their hands full managing the divisions in their own party on the issue. But if they can, analysts of both parties agreed, the Democrats may be left mute.

"We've always been vulnerable on arms control, and the Democrats were going to try to beat us up on it next year," said Eddie Mahe Jr., a GOP consultant and former staff director of the Republican National Committee. "I hope they try," he added. "Because now we've got them outside the tent on this issue. The Reagan approach of hanging tough has worked brilliantly, and the people can see that. The Democrats' wimpiness will haunt them."

A nationwide survey taken in October showed that even before the glow of this week's summit, many in the electorate had come to accept the logic of the Republican argument. By 43 to 34 percent, Americans said Republicans would do a better job of negotiating an arms deal, according to a survey conducted for a public interest group by the Democratic polling firm of Marttila & Kiley (and reviewed by Republican pollsters for balance). Throughout most of the decade, Democrats have enjoyed a lopsided egde on that question.

The poll also showed that while 48 percent of Americans now think the nation is spending too much on defense and only 15 percent think it is spending too little, 69 percent agreed that the military buildup was necessary.

This week's summit could reinforce that approval.

The clearest beneficiary among presidential candidates is Vice President Bush. The arms deal strengthens his chances of winning both the 1988 Republican presidential nomination and the general election, according to political observers. They say:It has given his campaign something it lacked: a compelling agenda for the future.

It has restored vitality to the Reagan presidency, which rubs off most on Bush.

It will strengthen Bush's case for continuity, especially if there is agreement on but not ratification of a treaty slashing strategic nuclear-armed missiles.

It could assure that foreign policy, Bush's strong suit, will be on the electorate's radar screen throughout 1988.

The peril for Bush, as Ann Lewis, a Democratic consultant, and others note, is that if the intraparty GOP squabble over arms control is bitter and protracted, it could leave the party divided for the general election and the voters turned off by the rawness of the rhetoric. Last week, Conservative Caucus chairman Howard Phillips fired a venomous shot, accusing Reagan of being a "useful idiot" for Soviet propaganda.

Phillips said Reagan is pursuing a "suicidal course" and vowed that Bush would be its leading victim. "Once you stomp on your base, you better have some friends in your new base. I'm not sure George Bush can count on Mikhail Gorbachev to deliver New Hampshire."

Phillips' warning raises the question of timing. In one view, the biggest danger to Bush lies in the potential for a fractious GOP nominating convention next August at which right-wing conservatives stage a plaform fight against their own president and presidential nominee, hoping to prevent another arms deal. Others argue if there is such a fight, Bush might actually benefit from taking on the right wing of his own party -- so long as he wins.

The more interesting question is whether the ideological chasm in the GOP can have much impact on the caucuses and primaries that begin late in January.

With the Senate not scheduled to vote on ratification of the INF treaty until March or April, and with Reagan not scheduled to to travel Moscow until late spring or early summer, it is not clear whether conservative fears over arms reduction will peak in time. It is also doubtful they will find a single presidential vehicle -- at least not initially. Rep. Jack Kemp (R-N.Y.), former Delaware governor Pierre S. du Pont IV and former television evangelist Marion G. (Pat) Robertson are all actively campaigning against the treaty, splitting a minority base at least three ways.

Sen. Robert J. Dole (R-Kan.), Bush's principal challenger for the GOP nomination, has a dfferent problem. He has refrained from endorsing the treaty, though he has made it clear he intends to support ratification in the end. Some think his straddle is symptomatic of a candidacy that is driven too much by the rhythms of the Senate. Others say Dole is carefully positioning hismelf to the right of Bush so that if the conservative challengers fall out of the race early, he can become the conservative standard-bearer.

It is a high-risk course, especially given that Dole's showdowns with Bush are likely to occur early. "He might wiggle his way through," said Mahe, "but he pays a price for vacillation now and it's not certain he'll ever be able to get the benefits."