There is a great deal of speculation in Washington, D.C., this week as to why President Carter has decided to leave the Rose Garden and go out on the campaign trail.

While everyone seems to be giving a different reason, I can reveal the true one today.

An unusually reliable source at the White House told me, "We decided to switch political strategy because of Rosie Ruiz, the lady marathon runner."

"What has she got to do with the presidential race?"

"Nothing. But as you know she claimed she ran the 26 miles in the Boston Marathon, and after being declared winner of the women's division, the officials took the crown away from her because no one saw her run."

"And Carter decided to leave the Rose Garden because of that?"

"We received information that Teddy Kennedy was going to go to the Democratic Convention in August and claim Carter had never run for reelection, and therefore was not entitled to be nominated."

"But Carter was running in the Rose Garden," I said.

"It wasn't enough. No one had seen him run in the states that had primaries. They saw Rosalynn running and Chip running and Miz Lillian running and Vice President Mondale running for him. But there isn't one piece of film that shows Carter himself. Of course, the president planned to cross the finish line himself, but we were afraid that Teddy's protest might hold up, and Carter could have been disqualified."

"Could the party do that to an incumbent president?"

"Nobody knows. The rules are unclear on how farr a candidate has to run to qualify for the nomination. The fact that Carter hasn't done any of the running himself hasn't hurt so far. But this Rosie Ruiz brouhaha has changed everything. Americans are starting to ask if it's enough to have surrogates do all the running for the candidate and then give him the gold medal. We felt that as long as Carter ran around the White House he would be qualified for the marathon. Obviously they don't feel that way in Boston, and it would be just like Kennedy, if he lost, to protest a Carter victory."

"But your Rose Garden strategy worked so well. We saw more of Carter running in the White House than we would have if he had been out on the road. Suppose Carter falters when he joins the other runners?"

"He only has to beat Kennedy, and he has such a commanding lead that we don't see any danger of hom slipping now."

"You mean he can join the race where it is now and not begin at the starting line?"

"Of course. The president has a few more miles to go, and even if he jogs, we don't see it as a problem. The important thing for Carter is just to show people he was willing to get into the race. As long as we have film of him running, the Kennedy protest won't mean a thing."

"How does the president feel about it?"

"He's looking forward to running somewhere besides the Rose Garden. He's always said that if he got into the race with Teddy he would whip his a--."

"It will be good to see the president on the road again," I admitted. "Particularly since he had nothing but bad news to give us when he was running around the White House. Maybe once he leaves, things will get much better." a

"They are much better. That's why Carter said he could afford to get out and run."

"Well, I said, "I think the president is doing the right thing. One more question -- if Carter is out on the road, who will be running in the Rose Garden?"

"The president has invited Rosie Ruiz to use it to get ready for her next race."