Jack Kemp
https://en.wikipedia.org/wiki/Jack_Kemp only held one elective office in his life--Congressman from the Buffalo suburban district. Here is the first
Almanac of American Politics (written in 1971) on how he (narrowly) won that district (the New York 39th, as it was then numbered) in 1970:
"From 1964 to 1970, the 39th's congressman was Richard D. (Max) McCarthy, the liberal Democrat who made national headlines when he exposed the Defense Department's stockpile of chemical and biological weapons. He would probably still represent the district if he had not decided to enter the Senate Democratic primary in 1970. His campaign was doomed from the beginning. McCarthy did attract some publicity for scuba-diving into the murky, polluted waters of the Hudson River, but when the votes were counted, he finished a poor fourth. He carried only the Buffalo area, and this he did with the help of Joe Crangle's Erie County organization, one of the few New York Democratic machines still capable of producing votes. After the defeat, McCarthy tried to get back into the race in the 39th district. There was talk that the Democratic nominee, Thomas P. Flaherty, would be slated for a judgeship, which would then permit the county Democratic organization to designate McCarthy as the candidate.
"But Flaherty refused to budge. McCarthy then tried and failed to get on the ballot as an Independent, and the attendant publicity among feuding Democratic politicians worked to the advantage of the Republican-Conservative nominee, Jack Kemp. Kemp was well-known, having been a successful quarterback for the Buffalo Bills...Kemp's recognition among the voters, his campaigning ability, and his articulate support of the Nixon Administration won him a narrow 52% victory..." (p. 578)
Suppose that McCarthy had not run for the Senate, or that after the primary, Flaherty had agreed to step down (in return for being slated for a judgeship) to enable McCarthy to run for the House seat again. Since even Flaherty almost beat Kemp, it seems very likely that the popular incumbent McCarthy would have done so, and would go on being re-elected through the 1970's. As it was, Kemp made this once politically marginal district (it contained both blue-collar and white-collar suburbs) safely Republican; his share of the vote rose from 52 percent in 1970 to 73 percent in 1972 and 78 percent in 1974.
If Kemp had not been able to win the congressional seat in 1970, it is not clear to me how he could have made a breakthrough into electoral politics--a successful bid for statewide office seems unlikely, because Kemp at the time was little known in New York state outside the Buffalo area. So suppose that Kemp's political career was cut short in 1970? In OTL in the late 1970's he became the chief popularizer of supply-side economics. Would it still have become GOP doctrine without him? Edwin Meese has denied that Kemp sold supply-side economics to Reagan, claiming that Reagan "even anticipated the 'Laffer curve'"
https://books.google.com/books?id=LYQcCgAAQBAJ&pg=PT82 and that he favored drastic tax cuts well before his January 1980 Los Angeles meeting with Kemp--"far from having to convert Reagan to anything, Jack was basically knocking on an open door."
https://books.google.com/books?id=LYQcCgAAQBAJ&pg=PT83
Kemp's biographers, Morton Kondracke and Fred Barnes in
Jack Kemp: The Bleeding-Heart Conservative Who Changed America argue that Meese and Martin Anderson somewhat underrate Kemp's influence: "As evidence, Meese cites the Reagan campaign's Policy Memo No. 1, drafted by Anderson in August 1979, which set forth the essence of Kemp-Roth without mentioning the bill itself...
"Still, Reagan's program had long been an amalgam of supply-side and old-fashioned Republicanism. He did not run in 1976 on a low-tax platform. In 1980, he did--specificaly on the program written and fought for by Kemp. His adoption of Kemp-Roth as a 'political deal' was unlikely because Kemp had already endorsed Reagan by the time of the LAX meeting. And Kemp's bill had been introduced years before Policy Memo No. 1...[John] Sears, also at the LAX meeting, confirms that Reagan had not formally embraced Kemp-Roth in November. 'We had some things about taxes, but not Kemp-Roth. In January we embraced it.'"
https://books.google.com/books?id=EwUbBQAAQBAJ&pg=PT53
(Apart from popularizing "supply side" tax cuts, Kemp's legacy seems more precarious today than it once did. He was the chief proponent of the idea that the GOP had to reach out to minorities to win--an idea harder to maintain after 2016...)