It might have been President Gerald Ford out in Detroit leading the Republicans next week, some think -- except for two very critical blunders. One, says Robert L. Hartmann, longtime aide and adviser, was Ford's failure to choose Nelson Rockefeller as his running mate in 1976. The other was the former president's refusal to clean what Hartmann calls "Nixon's Praetorians" out of the Ford White House.

For obvious reaons, there were few, if any, recognizable "Praetorians" (the names is taken from the ancient Roman palace guards) around last night for the publication party celebrating Hartmann's new book, "Palace Politics: An Inside Account of the Ford Years." The "Praetorians," Hartmann says in the introduction to his book, were a close-knit bunch of Haldeman-Haig subordinates "who nearly wrecked the Republican Party."

"No discrimination," insisted Hartmann, claiming the guest list was drawn up by Oakley Hunter, board chairman and president of the Federal National Mortgage Association, in whose stunning Wisconsin Avenue offices the party was set.

Still, there were no Alexander Haigs or Donald Rumsfelds or Henry Kissingers or Helmut Sonnenfeldts to be seen among the crowd of 180. There were, however, such Ford faithfuls as Philip W. Buchen, L. William Siedman, Gwen A. Anderson, Robert Orben and John Calkins, drinking cocktails and partaking of sliced roast beef and miniature crab cakes.

In fact, Hartmann, who had been counselor to Ford with Cabinet rank, seemed quite certain that his candid portrayal of Rumsfeld, a former Ford chief of staff, would not make Rumsfeld angry.

"I'm also somewhat sympathetic," said Hartmann, "because President Ford gave him the impression that he had some authority, and it turned out that he didn't."

Some even had postscripts to Hartmann's published perspectives of Gerald Ford's 895-day presidency. One was that it was Rumsfeld who built the conflict between then-secretary of defense James Schlesinger and then-secretary of state Henry Kissinger.

"And you know who would win that battle," said the observer, noting that once Rumsfeld got Schlesinger's job, "he was really out of his element."

It was a name that four years hadn't erased from Republican minds any more than they had Gerald Ford's, though this time the Ford talk was in connection with the No. 2 spot on Ronald Reagan's ticket.

"Oh, no," said Arthur Burns, former chairman of the Federal Reserve Board, to questions about a Gallup Poll favoring Ford as Reagan's running mate. "Under the Constitution it says electors cannot vote for a president and a vice president from the same state."

"He said he wouldn't want to move from California," Phil Buchen reminded a guest with a grin, "but that could be corrected by having President Reagan move."

The GOP platform committee's action yesterday repudiating the party's historical support of ERA was met with muted despair by Gwen Anderson, who was deputy executive assistant to Hartmann under Ford. "Being a cold-hearted politician, I simply feel they have to be very careful of these special-interest groups [anti-ERA] if they want broad-based appeal."

Said Hartmann: "There are two mistakes here. One is the position itself. Any politician in this particular time has to be for ERA or he'll antagonize enough people and he'll lose. But the second and worse thing, is to repudicate a position he's always taken.

"Like a president picks a vice president -- Nelson Rockefeller -- after careful thought and study, not in the heat of a convention. And then after two years he decides maybe all his reasons about the best possible man are not valid anymore and we have to have another man. The change is fatal -- the switching itself is fatal."

That, said Hartmann, was where Ford lost the election, saying in effect that "my judgment was bad before and I'm having second thoughts about it." What happened, he said, was that Ford lost Rockefeller's people, he lost moderate-to-conservative people who don't like to see somebody waver.

"And he didn't win one Jesse Helms -- not one."