Both FDR and Truman, for each of whom Rockefeller worked, urged Rocky to become a Democrat. For one reason he declined Truman's offer, see Richard Norton Smith,
On His Own Terms: A Life of Nelson Rockefeller, pp. 216-217:
"His second departure from Truman's Washington was amicable enough, the president expressing regret that he couldn't appoint Nelson to another post in his aging administration. He also urged Rockefeller to become a Democrat. In later years, much would be made of Nelson's failure to heed the presidential counsel. This overlooked one crucial factor, quite apart from familial loyalties: At the start of 1952, with the Korean War in bloody stalemate and China over-run by Mao Tse-tung's Red armies, Truman and his Democratic brethren were widely discredited. Adding to public angst, Soviet spies had shattered the West's nuclear monopoly. Fears of domestic Communist subversion were on the rise. Petty White House scandals added to a pervasive sense, after twenty years in power, that Truman's party was out of touch. In the twilight of his presidency, Truman enjoyed the lowest approval ratings since the dawn of modern opinion polling. Under the circumstances it defied logic for Nelson, nearing the end of a long, often frustrating voyage under Democratic captains, to lash himself to the mast on the eve of a Republican restoration. Moreover, by 1952 the GOP's Eastern Establishment, long synonymous with the Rockefeller family, felt confident that its brand of progressive conservatism, shorn of isolationist barnacles and broadly accommodating to the welfare state bequeathed by FDR, might at last command a majority of restive voters. Especially if the face of Republican realignment wore a grin as wide as the Kansas prairie...."
https://books.google.com/books?id=fzeODQAAQBAJ&pg=PA216
Of course while this may explain why he didn't become a Democrat in early 1952, it doesn't answer why he had never become one. Rockefeller himself once gave this explanation:
"I used to play tennis at Foxhall Road [in Washington] with [Henry] Wallace and [Rexford] Tugwell, and I was always uncomfortable with their very leftish views. The way I saw it, if I became a Democrat, I'd always be in the position of holding the party back, whereas if I stayed a Republican, I'd be pushing the party forward." (Cary Reich,
The Life of Nelson A. Rockefeller: Worlds to Conquer, 1908-1958, p. 689)
So it's not clear to me that Rockefeller ever seriously considered becoming a Democrat. It's not just the Republicanism of his family, but the fact that in the 1940's and 1950's, a reasonably progressive "Eastern Establishment" Republicanism seemed to have a good chance of prevailing, both in the GOP and in the country. (And the Democrats after all, contained elements Rockefeller may not have particularly liked: leftists, big city bosses, southern segregationists.)
There was IMO one chance for Rockefeller to run on a Democratic ticket, but it was much later, in 1968. To quote an old post of mine:
***
Former Massachusetts governor Endicott Peabody, a summer neighbor of Nelson Rockefeller's (and a staunch Democrat) tried to draft Rocky as VP on Humphrey's ticket in 1968, calling on Rocky to take part in a grand alliance of Humphrey, Kennedy, McCarthy, Rockefeller and Romney supporters to prevent the victory of "a leadership outright opposed to your policies." "Humphrey himself called to make the case for a coalition government." Richard Norton Smith, *On His Own Terms: A Life of Nelson Rockefeller*, p. 540. See also Marianne Means' column of September 4, 1968:
http://news.google.com/newspapers?n...8cfAAAAIBAJ&sjid=iNgEAAAAIBAJ&pg=4589,6345352
Yes, it may seem unlikely that Rockefeller accepts, but after 1964 and 1968, maybe he concludes that he just has no future in the national GOP and that he is closer philosophically to Humphrey? Anyway, while normally "nobody votes for the veep",
http://www.slate.com/…/20…/06/nobody_votes_for_the_veep.html could it be different this time, given the closeness of the election and the possibility that Rocky's presence on the ticket could appeal to some moderate-to-liberal Republicans (and moderate independents) who may have been reluctant Nixon voters in OTL?