Ronald Reagan is riding the cushion of a post-debate in the final drive for the presidency, according to the private assessments of both camps, and President Carter's strategists are now working on ways to try to nullify the Republican's newly won advantage.

Reagan apparently accomplished a major objective in Tuesday's debate which was to reassure a significant portion of the electorate that it would not be a risk to have him as president, Carter campaign officials concede. They base this finding on the results of a new national survey by Carter pollster Patrick Caddell.

But both Carter and Reagan strategists now also say that there is no certainty that Reagan's debate afterglow will last until Election Day. Officials in both camps noted that Reagan also received a quick boost in polls taken immediately after his debate with independent candidate John B. Anderson on Sept. 21, but that this boost disappeared after just five days.

This time, the Carter officials hope to use a new round of media advertising to extinguish the afterburn from this second Reagan debate performance. One commercial will highlight the fact that in the debate Reagan denied having opposed efforts to stop the spread of nuclear weapons in nonnuclear countries, when, in fact, he actually has taken just a stance.

This Carter ad, scheduled to run tomorrow, will show the president saying in Tuesday's debate that Reagan had once said that nuclear nonproliferation was "none of our business." Then the ad will show Reagan denying, in that debate, that he had ever said such a thing. And finally, according to Carter campaign sources, the ad will show Reagan saying in Jacksonville, Fla., last January that he did not believe the United States should stand in the way of other countries developing their own nuclear weapons because "I just don't think it's any of our business."

Reagan poll-taker Richard Wirthlin had concluded earlier, based on the surveys of others, that Reagan had helped himself most in the debate. Now he has data from his own polls that tells him this was indeed the case. In a nationwide sampling of 1,500 people, 41 percent said that Reagan had won the debate and 26 percent said Carter won.

Asked if Reagan had increased his chances of being elected by his debate performance, 62 percent said yes and 21 percent said no; asked if Carter had increased his chances of being elected, 45 percent said yes and 34 percent said no.

And finally, when Wirthlin asked those interviewed to rate the debate performances of the two candidates on a scale of 1 to 10, Reagan finished with a 4.81 rating and Carter with 4.44.

Carter officials are somewhat baffled by the findings of their own survey, according to sources in the president's campaign. For although their poll showed that Reagan had achieved a slight nationwide benefit from the debate, these figures were not borne out by some regional samplings. In Mississippi and Oregon, two diverse states were the contests have been very close, Carter actually improved his standing a bit in Caddell's postdebate survey. Carter wound up after the debate to be narrowly leading Reagan, although by margins that are still within the range of the sampling error.

Carter campaign officials contended that nationwide attitudes may well have been influenced by instant media analysis -- and especially by a highly controversial ABC News call-in, where viewers were urged to telephone their preference as to who won the debate into national "Carter" and "Reagan" switchboards. There were a host of problems with the unscientific experiment, but the network went ahead and made the result, which showed a 2-to-1 Reagan debate victory, part of their instant analysis that followed the debate.

Said one Carter official, in describing the apparent Reagan edge that actual polls, including the president's own, have since shown: "It's a media bump."

In the last 10 days of the campaign, the Reagan campaign will apparently vastly outspend the Carter campaign in the crucial category of television advertising. Reagan will spend $6 million on media, most of it in television ads, according to the Republican nominee's media consultant, Peter Dailey.

Carter will spend about $4 million over the same period, according to his media adviser Gerald Rafshoon. In addition to the television ads aimed at undoing the damage caused by the debate, there will be another ad aimed at boosting the turnout among Democratic voters, reminding them of the need to vote Democratic once again.