POD of 1980, make it happen. Apparently he considered it in 1988 and 1996 but never followed up.
Well, apparently some people saw him as a viable VP candidate in 1980. Not sure if Reagan himself seriously considered him, but you could always have Rumsfeld take the same path as Bush. VP under Reagan for 8 years and then get swept into office in 1988 on Reagan's popularity.
There was also a Senate election in Illinois in 1980. He could run for that and then go from there.
Years later, when Rumsfeld's old friend Richard Allen revealed that he had persuaded Reagan to pick George Bush as his vice presidential running mate and supplied Bush's phone number (Bush had given up hope by the time Reagan called and was getting drunk with James Baker), Rumsfeld called Allen and asked, "Why didn't you tell him to pick me?" To which Allen generously responded, "Because I didn't have your number."