EN ROUTE WITH CARTER - A hint that Jimmy Carter's self-confidence is now approaching spill-over level's after dangerous depletion last summer came in a presidential bull session in the VIP cabin of Air Force One returning from an all-day campaign swing.

Usually when Carter boards the presidential plane, he quickly takes refuge in his private quarters. Not so on the night of Oct. 28. Buoyed by good crowds and his own above-normal rhetoric, the president felt the urge for company and political gossip with Democratic National Chairman John White, political side Tim Kraft and others.

The uncommon act of shooting a little bull with the boys on the long way home-uncommon for Jimmy Carter-was a subtle signal that Carter now feels he has weathered a period of grave danger to his presidency. Ten weeks ago Dr. George Gallup's national poll found only 36 percent crediting the president with "strong leadership qualities."

Whether this new confidence is a false drawn or represents what one aide calls the president's "feeling that he finally has his hands on the levers," time will tell. But in these final days of the 1978 campaign, Carter exudes an image of presidentially not visible in the darker days of summer.

He brushed aside worried Liberal Democrats in Massachusetts who did not want their hostility for Edward J.King, the conservative Democratic candidate for governor, to be trained by a presidential blessing for King. Carter wisely insisted that the Democratic tent is abroad enough for a King or two. His decision was more than indicated in City Hall Square in the town of Lynn, when one of the most exuberant crowds of the campaign season gave him louder, longer applause than it gave Sen. Edward M. Kennedy.

Likewise, that same evening he passed up an easy chance to milk a partisan dinner crowd in Portland's Stevens Street Armory. Stopped three times by applause during his opening six sentences, he shocked his political aides by saying, "I want to talk soberty and frankly tonight. I don't come up here to waste my time."

There followed a long extemporaneous talk on the "compassion" of the Democratic Party, more sermon than stump talk, centering on the uphill re-election campaign of Sen. William Hathaway. Hathaway is a likely loser on Nov. 7 to Republican Rep. William Coben. The rhetoric, echoing Carter's early presidential campaign themes of 1973 and 1975 and 1976, was stripped of applause lines, but the packed crowd clearly accepted the switch of mood from stump to [WORD ILLEGIBLE] "He's really cocking tonight," one often-caustic White House aide muttered.

Here was a beachmark for the restoration of Carter, a return to the old political evangelism with enough self-confidence, as one party operative told us, "to bring the crowd down from exuberance instead of building it up even higher, and make them like it."

A similar confidence has been on display recently inside the Oval Office. Three days before his Oct. 24 anti-inflation speech, the president rebuked his economic advisers for letting him publicty promise quick inflation relief from his proposed wage and price quideliness while those same advisers were privately warning that quick relief was impossible.

Demanding the "truth" on what the new program might achieve, and how long it might take, Carter ordered up a carefully hedged speech that had as many cavaeats in it as promises of quick relief. "The defensive quality in Carter is down," one White House aide told us. "He's acting more like he used to, and he is not constantly agonizing about what he ought to be doing from one minute to the next."

The 1978 campaign trail, which a few months ago looked forbidding for a president few candidates wanted anywhere near them, is where the confidence shows up most. Testing his themes for 1980, Carter justifiably globeries in his role as Middle East peacemaker and reminds his audiences that "not one drop of blood" has been shed by U.S. servicemen since he got elected.

In fact, Jimmy Carter may not have changed any votes at all in his extensive campaigning this fall among voters who are singularly disinterested in politics and who have turned ominously sour on government at all levels and the politicians who run it. But he may have done something more important for Jimmy Carter, proved to himself that, for now at least, he has restored his presidency.