George Bush became Ronald Reagan's midnight choice for vice-president after a frantic day of negotiations failed to persuade former President Gerald R. Ford to accept the slot.

The choice of Bush, Reagan's most persistent challenger in the primaries, stunned delegates who had been reveling in the prospect of what they called "a dream ticket" that would drub the Democrats in November.

Hoping to put the best possible interpretation on as extraordinary a day of maneuvering as the political conventions have ever seen, Reagan broke precedent and came to the convention hall shortly after midnight to explain the choice to the delegates and the nation.

Reagan said he wanted to "straighten out . . . the rumors and the gossip," and confirmed that he had been negotiating with Ford.

But the nominee said, "He and I have come to the conclusion . . . that he can be of more value as the former president, campaigning his heart out, as he has pledged to do, than as a member of the ticket."

Then he told the delegates he had turned to Bush, and Bush "told me that he can enthusiastically support the platform across the board."

Bush had been the odds-on favorite of the delegates and senior Reagan staff until this morning, when Ford's last-minute wavering invited Reagan operatives to step up their bid for his services.

As part of the negotiations, according to informed officials, former secretary of state Henry A. Kissinger, former chief economic adviser Alan Greenspan and former White House counselor John D. Marsh Jr. secured an agreement from Reagan aides that would have made Ford "the chief operating officer" in a restructured Reagan White House.

The "shared presidency" concept, according to some officials, would have given Ford special authority in the area of national security policy.

But while Ford told a national television audience as the evening convention session began that "pride" would be no barrier to considering such a deal, in the final moments of decision, he said no.

The Ford decision changed the mood in Bush's hotel suite from bitterness and despair at the apparent loss of the consolation prize to sudden jubilation.

Bush later told reporters the call from Reagan inviting him to be the running mate "came out of the clear blue sky . . . he was most gracious in the invitation, and of course, I was very pleased."

"I feel honored . . . an enormous compliment," the former congressman, ambassador and Central Intelligence Agency director said. "I did tell him I would do what all Republicans should do, enthusiastically support this platform and I told him I would work, work, work, work. . . ."

While Bush was understandably charitable toward Reagan's waverings, it was not clear what effect the nominee's performance would have on voters weighing his qualifications for the presidency.

During the hours of the courtship of Ford, Reagan aides made no effort to hide the fact that the presidential nominee was reluctant to accept Bush on his ticket because of his doubts about Bush's campaigning ability and his fitness for the presidency.

Private polls taken for Reagan showed Ford as the only person who would add strength to the ticket.

Conservatives who had steeled themselves to accept Ford on the theory he would assure Reagan's election were distinctly less cordial to the prospect of Bush -- who has differed with Reagan by supporting the Equal Rights Amendment and opposing a constitutional amendment to ban abortion.

But Bush's brief speech preceding Reagan's nomination roll call was as lavish in praise of his erstwhile rival as it was devoid of any sentiments that Reagan's backers could find remotely objectionable.

Bush, the last to yield to Reagan in the primaires, called the Californian "a winner for our country and our party," who would end "four years of Jimmy Carter's fumbling, incompetent leadership."

"If anyone wants to know why Ronald Reagan is a winner," Bush said, "you can refer him to me. "I am an expert on the subject. He's a winner because he's a leader . . . his message is clear. His message is understood."

If that is true of Reagan, then it was not true tonight of Gerald Ford, who led Reagan to believe he was ready to join an unprecedented partnership with a former president as the number two man -- only to scuttle it at the end. w

There were unconfirmed reports that Ford had asked specific authority to veto Reagan's choices for secretary of state, secretary of defense, and director of the Office of Management and Budget. Those demands were unacceptable to Reagan, according to these reports, and the deal collapsed.

But Reagan, in a brief television interview at Joe Louis Arena, denied that they had fallen out over the terms of Ford's charter as a potential vice president.

On the convention floor, however, rumors abounded that Reagan had balked at the implicit suggestion that taking Ford would also have meant accepting Kissinger in his old job as secretary of state.

At first, Reagan had seemed responsive to the suggestion from the Ford intermediaries that the White House could be restructured to make Reagan the "chief operating officer."

Reagan has often said he believes in the "chairman of the board" approach to executive leadership, and his spokesmen said any constitutional problems could be solved by making it clear that ultimate responsibility for every decision would remain with the president.

In extraordinary television interviews this evening, Ford insisted on "substantive assurances" from Reagan that he would "play a meaningful role on substantive decisions across the board" and "not just a ceremonial role" in the Reagan administration.

But he and his wife, Betty, said they would have no "pride" problems in returning to Washington as the number two couple in a capital where for 29 months they were the president and first lady.

At these indications of Ford's willingness to come aboard, Reagan lieutenants became less cautious in telling reporters that the former California governor had never been reconciled to running with Bush.

On the convention floor, as rumors of a Reagan-Ford alliance circulated, many delegates said they thought the combination would give Republicans an advantage that the Reagan-Bush partnership could not provide.

Tennessee Gov. Lamar Alexander said, "I think it's a great idea. I know that it would carry Tennessee, and that was Carter's second-strongest state in 1976."

But even before the word spread that the "dream ticket" had fallen through, misgivings were being expressed. Sen. Paul Laxalt of Nevada, Reagan's campaign chairman, said the Democrats might criticize the 69-year-old Reagan and the 67-year-old Ford as "two old men." But he added, "It's a question of what you call old. A lot of people 40 or 50 years old in this country could keep up with either one of them."

Others worried about the political and constitutional implications of what was, at least in its initial presentation, a somewhat vague "shared presidency."

Sen. Jacob K. Javits (R-N.Y.) said, "I don't think it would work. I can hardly conceive of a co-executive. Someone has to be boss."

Sen. Richard Lugar of Indiana, himself a vice presidential contender, said: "I don't think it presents any constitutional problem, but it has the potential for a real political problem. I suspect on reflection that they might decide having two people serving as president won't work."

But concerns about the workability of the concept were subordinated, for a time at least, to political eagerness to see the partnership sealed. Sen. Jesse Helms (R-N.C.), a conservative who had threatened to protest the choice of Bush as a running mate, had said that Ford's nomination 'would go through with a hoot and a holler."