How long would Republican dominance last without the Great Depression?

[Cashew]

how long could the post civil war map last?

[RINO Tom]

I mean, was it really the same map?  I'd say certainly not.  However, given the spirit of your question, I'd say it was already faltering by the mid 19-teens and definitely by the late 1920s.  I remember reading an article about how Indiana Democrats won a plurality of the Black vote in 1928 because the Klan gave a blanket endorsement to local Republicans.  Al Smith certainly gave the Democrats a different flavor.

I'd say by the time WWII was drawing near (assuming it broke out in the same fashion, which is unlikely given the effect of our depression on the world), Republicans would eventually lose support among pro-war people, maybe?  They certainly would have lost support from some areas of the North for starting to embrace freer trade (which they did anyway) in order to keep their corporate funders happy.  I mean, the GOP had also been trying to nab White Southerners since ... well, shlt, like right after Reconstruction.

[Southern Senator North Carolina Yankee]

The loss of GOP dominance was largely driven by generational change. The Depression merely added depth to the blowout, but the timing would have been about the same.

[Chunk Yogurt for President!]

Until the next economic or foreign policy catastrophe.

[OSR stands with Israel]

I mean, was it really the same map?  I'd say certainly not.  However, given the spirit of your question, I'd say it was already faltering by the mid 19-teens and definitely by the late 1920s.  I remember reading an article about how Indiana Democrats won a plurality of the Black vote in 1928 because the Klan gave a blanket endorsement to local Republicans.  Al Smith certainly gave the Democrats a different flavor.

I'd say by the time WWII was drawing near (assuming it broke out in the same fashion, which is unlikely given the effect of our depression on the world), Republicans would eventually lose support among pro-war people, maybe?  They certainly would have lost support from some areas of the North for starting to embrace freer trade (which they did anyway) in order to keep their corporate funders happy.  I mean, the GOP had also been trying to nab White Southerners since ... well, shlt, like right after Reconstruction.

Hoover won more electoral votes then Harding did and unlike Harding also won half the south and like harding dominated the north.

[mianfei]

Until the next economic or foreign policy catastrophe.

Almost certainly that is the answer. It is true that Al Smith in 1928 mobilized many of the new generation of “white ethnics” whose ancestors had come over from Southern and Eastern Europe, but whether they would have continued to go to the polls without him and without a depression is not clear.

For all its anti-democratic tendencies, the “System of 1896” was extremely effective at improving living standards for the majority of Americans, and without Smith (and La Follette, who had mobilized many voters who turned to Smith in 1928 both in the 1924 election and with his family’s endorsement of Smith after he died in 1925) there is always the possibility of continuing declines in political interest among the majority without a crisis to arouse them.

[Skill and Chance]

Hoover would easily win reelection (and historical memory of him would be on par with Reagan), but gains the Northern cities should give them the open seat in 1936 with something like the 2012 PV margin.  After that, I think they win 1940 and 1944 if WWII still happens and then lose in 1948 without historical memory of the Depression (and something tells me the economy would crash badly by 1950ish if there was no 1930's crash).  16-20 consecutive years in power is about the historical limit for either party. 

[Kingpoleon]

Democrats win in 1936 with Charles W. Bryan on top and Paul McNutt as his running mate. Bryan puts Alfred Smith, Albert Ritchie, C. Ben Ross, and Elmer Benson in his Cabinet. Ritchie represents the conservative wing of the party, Benson represents progressives, Ross represents agrarian and Western interests, and Smith is a symbol of both the establishment as well as a new voter base: ethnic whites and Catholics. This “Cabinet of Rivals” ensures that no one wing can establish a strong enough position, keeping Bryan in charge of the Democratic Party.