It would be wrong to say that Ronald Reagan's presidential campaign owes everything it is today to the political director of George Bush's campaign.

Just its rent.

In part because of the grand old fraternity within the Grand Old Party, and in part because so often in Washington politics the unforseen becomes probable and the unplanned becomes policy, it has turned out that Bush's campaign adviser, David Keene, is now Ronald Reagan's campaign landlord.

Every month, the Reagan campaign pays $1.200 rent to a partnership headed by Keene.

"We didn't plan it this way," says Keene. "It just sort of happened."

Back in the days when Keene was a major figure in the Reagan political command, he put together a partnership with a number of his friends on the right-including several other Reagan aides-and bought a three-story Victorian, tan brick building at 809 Cameron St. in Alexandria.

Keene became the managing partner, and took an office in the building. Then Keene and his Reagan friends came to a political parting. Reagan did not have a major role in mind for Keene in his 1980 campaign (just political liaison with Congress), and, meanwhile, George Bush was asking Keene to be the political director of his presidential campaign. So Keene switched his political alliance, but he kept his business alliance.

At about thetime Keene moved out of the office building on Cameron Street, the Reagan people moved in. "I joined Bush and moved out and the Reagan people needed space and moved in," Keene says sitting in his new office in the Bush headquarters 12 blocks away from the building he owns.

The Bush camps and the Reagan camps found themselves to have common real estate interests. Both have headquarters in their candidate's home state (Bush in Texas and Reagan in California). And both were looking for a cheap alternative to the high-priced downtown Washington real estate.

Now Reagan shares his Alexandria headquarters building with two other tenants: one is a psychiatrist and the other is a branch of the enterprise of Richard A. Viguerie, specialist in conservative political fund-raising and mailing.

And Keene shares his monthly rental payment with his partners. His share is 20 percent, he says, and the rest is divided among eight or nine others, including Reagan's finance chairman, Lyn Nofziger; Reagan's Northeast cordinator, Roger Stone; Thomas Winter, editor of the conservative Human Events magazine; Terry Dolan of the National Conservative Political Action Committee, and Paul Russo, who took the job that Keene did not want as Reagan's liaison for Capitol Hill.

Over at the Federal Election Commission, an official found the business ties between the Bush adviser and the Reagan camp interesting and even amusing, but no violation of any regulation.

And, meanwhile, out in California, at Reagan's main headquarters, Lyn Nofziger said the Reagan campaign couldn't be happier than to be paying its monthly rent to Bush's adviser, Keene.

"We figure we're getting fair value received." Nofziger said, laughing. "We think we owed it to him after all the nice things he has said about us since he went to work for George Bush."