The Lodge Bill Becomes Law in 1890

[Skill and Chance]

The Lodge Force Bill would have required direct federal judicial supervision of elections in any states where citizens alleged discrimination.  It was broadly similar to the Voting Rights Act of 1965 and strongly supported by Benjamin Harrison: http://en.wikipedia.org/wiki/Lodge_Bill  How would subsequent American history have changed if it had passed the Senate?  Keep in mind that MS/LA/SC were outright majority black in the 1890's with MS and SC close to 60%.  Black Republicans would presumably take control of those state legislatures within a few years under full turnout.  GA, AL and FL were all between 40-50% black, which suggests competitive elections with full civil rights enforcement.  In other Southern states with a significant white majority, Democrats would presumably remain in control.

[TNF]

The main beneficiaries of this will probably end up being the Populists, given that theirs was a coalition of poor whites and blacks in the South and that anything that keeps blacks voting probably helps preserve that coalition and prevents the South from slipping into one party despotism.

Maybe you end up with a three party system emerging (at least until the Populists can successfully displace either the Democrats or the Republicans; my money is on them winning enough of the vote in the West and South to turn the Democrats into a regional Southern party), in which the GOP dominates on account of the divided opposition parties.

[Skill and Chance]

One thing that is clear is that Harrison would to win the electoral college outright in 1892 or the overwhelming Democratic majority in Congress would strangle the Lodge Bill in its cradle.  It is viable to get Harrison to a narrow majority by flipping the Deep South states with the worst vote suppression records and a couple of close and historically pro-civil rights Northern states:

One of the most surprising aspects would be the prompt end to segregation and Jim Crow laws in the Deep South states while these practices would presumably continue for many decades in the peripheral South where the 20-35% black vote wouldn't hold sway.  If he won in 1892, Harrison would get 2 more SCOTUS appointments by 1896, so segregation would presumably still be upheld, but perhaps by 5/3 instead of 7/1.  And the case wouldn't come from Louisiana. 

Then comes 1896, which with Harrison as president, the Republicans are sure to lose.  But do they lose to the Democrats or to Populists who have moved to the social right?  Harrison probably moves further left on silver in his second term to peel off some Western Populist support, so the Democrat is probably pro-union but also pro-gold standard.  Supposing a Democrat wins in an election with federally enforced black enfranchisement, he probably does this by dominating the Midwest and peeling off NYC metro support over Silver:

From here, Democrats probably hold the presidency almost continuously until the Depression.  Given 6 years of truly free elections to consolidate black Republican power, I don't ever see Democratic machines being reestablished in LA/MS/SC/AL/GA, but the window for any further progress would close as Democratic-appointed judges soften their enforcement of the Lodge Bill.  There would probably be substantial black migration into the Deep South states where they would have political power and full civil rights.  Racist whites would presumably migrate out of these states after realizing that they could not muscle their way back into power.  Republicans have to wait until the Depression to get full control of Washington.  With more black political power to advocate against it, segregation ends in the 1930's either by constitutional amendment or by Republican SCOTUS appointments.  1932 would look something like this in that world: