As far as the chairman of the Democratic Congressional Campaign Committee is concerned, the Republicans can have Rep. Larry McDonald, the conservative Georgia Democrat, "whenever they want him."

So far as the White House is concerned, McDonald would be welcome if--and perhaps only if--other Democrats were switching with him.

As far as McDonald is concerned, he's standing pat, at least for now.

That is how things stood last night after Rep. Tony Coelho (D-Calif.), presidential political assistant Lyn Nofziger and finally McDonald had sought to unravel the mystery of the latest rumored party-jumping Democrat.

It began when Coelho, at a briefing aimed at impressing reporters with the strides the Democratic Congressional Campaign Committee has made since he became chairman at the start of this year, expressed doubts that there would be many more Democrats following Reps. Bob Stump of Arizona and Gene Atkinson of Pennsylvania in the switch of allegiance.

Coelho said he had recent reassurances that two rumored bolters, Reps. Ronald M. Mottl of Ohio and Richard C. Shelby of Alabama, would stay with the party. As for the third, McDonald, Coelho said he guessed the White House could convert him "whenever they want him."

He said it without regret. McDonald, a fourth-termer who lists himself as a leader of the John Birch Society, is best known for filling pages of The Congressional Record with right-wing tracts and sponsoring amendments aimed at halting trade with communist countries and any form of government aid to homosexuals.

Coelho said that he understood Nofziger was claiming to have McDonald in his pocket and he would not want to dispute his fellow-Californian.

Nofziger told it somewhat differently. He said a member of his staff had talked with McDonald and "he's one you would logically expect to switch," but said he had not confirmed that for himself.

As for the timing, Nofziger said there was really no rush. "Rather than bringing one guy by himself," he said, "we'd like to bring two or three in quick succession--so it could have a little more impact."

That left it to McDonald, who said, through his press assistant, Tommy Toles, that he is "not switching, period," despite his belief that the national Democratic Party is "too far left."

That matter aside, Coelho radiated confidence that the Democrats were on their way back from their 1980 defeat. He said his committee had raised $2 million this year, more than it had ever done before, if still somewhat short of the $34 million the counterpart Republican committee has taken in.

He said that Democrats would gain 20 seats in the House if the election were being held now and would gain at least 10 seats next November, even if the economy improved by then.

His confidence, he said, was based on better candidates turning out for the Democrats since the economy began to turn sour last summer and on the belief that Republicans had some "soft incumbents" who had sneaked in on President Reagan's coattails.

Coelho said all that with the same confidence with which he said that McDonald would soon be a Republican. As they say on television, only time will tell.