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Senator Irving Ives, a generally liberal Republican, came very close to defeating Averell Harriman for the New York governorship in 1954--it was 49.61 percent for Harriman, 49.40 percent for Ives. http://www.ourcampaigns.com/RaceDetail.html?RaceID=94439
Suppose Ives had won--would this have blocked the political rise of Nelson Rockefeller? If Ives decided to run again in 1958, Rockefeller would have no plausible basis but personal ambition for challenging him in the GOP primary. Even if Ives decided to retire (as he retired from the Senate in OTL in 1958) Rockefeller's path to the governorship could have been much more difficult, with no unpopular Harriman administration to run against. "Time for a change" sentiment would be directed against the GOP, which in this ATL would have controlled the governorship for sixteen years. And don't forget that 1958 was a heavily Democratic year almost everywhere outside New York in OTL.
Granted, given his wealth and personal attractiveness, and given the divisions of the New York Democrats ("regulars" vs. "reformers") one could not rule Rocky out. But it could be a lot harder for him than in OTL.