If he was choosing not to have Rockefeller on the ticket, I’ve always been confused as to why he didn’t choose Reagan. I know that was a contentious, bad-blood primary but it would’ve United the party behind Ford at the very least.
I feel a similar way about Hillary Clinton not choosing Bernie Sanders, but I was alive for that one so it’s easier to comprehend.
It wasn't Ford's choice; Reagan himself opted out as a possible running mate.
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Due to the resignations of both Agnew and Nixon, Ford was not elected President, he was only elevated to it. This was part of what led to a long and bitter struggle for the Republican nomination against Reagan.
Despite Ford being the incumbent, he had the pardon of Nixon on his back, and Reagan was a popular California governor who grabbed a large chunk of conservatives. This led to a back-and-forth where Ford had a winning streak up to North Carolina's primary and then Reagan had the momentum leading up to the convention.
Reagan took the unusual step of announcing his running mate before anything was clinched (Sen. Richard Schweiker, a more liberal Republican); the Reagan campaign proposed a rule having Ford follow suit, and there was some public pressure for Ford to do this (the VP process was ongoing but he publicly said he was "too busy" with the campaign in general to settle yet on a running mate).
Rockefeller, Ford's VP at the time, said Reagan was out of the question because someone tending right in a campaign couldn't go even farther right on a VP choice. Contrary to Rockefeller, Ford still had Reagan in mind but at that time Reagan already said there was "no way" he would be Ford's running mate.
A later note was sent to the California delegation was even more explicit, that there was
...no circumstance whatsoever under which I would accept the nomination for Vice President. That is absolutely final.
When Reagan gave his speech following Ford's at the convention, he endorsed the platform of Republicans but not Ford himself.
Whether it is different this time than it has ever been before, I believe the Republican Party has a platform that is a banner of bold, unmistakable colors with no pale pastel shades.
Reagan did agree when Ford asked for campaign assistance, most particularly advice on a running mate. Dole was being considered as a way to sway the same conservatives who voted for Reagan, as the current vice president (Rockefeller) was liberal, and Reagan gave his endorsement to Dole. As Reagan himself noted, a more liberal choice would have lost the South:
...a lot of Republicans might not work for him. The balance of the country is in the Sunbelt, and that's where the future of our party is.
On the other hand, Reagan declined other requests from the Ford campaign to speak in particular states on his behalf. None of Reagan's refusals went into detail why he refused the VP slot (other than "second place is not for me"), but likely he already had in mind another eventual run at the top of the ticket, given his definitive nomination for the 1980 race.
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Ronald Reagan's 1976 address to the RNC can be found here.
I referred to these documents from the Ford Library, and the books:
Crain, A. D. (2009). The Ford Presidency: A History. United Kingdom: McFarland, Incorporated, Publishers.
Mieczkowski, Y. (2005). Gerald Ford and the Challenges of the 1970s. United States: University Press of Kentucky.