Here is my analysis on this issue.
North Eastern Republicans were long frustrated by the fact that Democrats had effectively neutralized the black vote in the South and other areas through tactics like "Grandfather clauses" and "Poll Taxes", etc.
The requirement to pay taxes in order to exercise the voting franchise has been with the nation since before it's founding and was considered a bedrock principle of American government.
The North Eastern (Liberal) Republicans saw this as causing them a loss of political power because the requirement to pay taxes was costing them approximately 10 million black votes that they otherwise would have, and so they created the 24th amendment which was intended to allow non tax payers to vote.
Now the amendment was sold as banning "poll taxes" because many people saw them as unfair and wrong, but it did in fact eliminate the requirement to pay any taxes.
The Liberal North Eastern Republicans believed this would insure their continuing power in the future because it would give them an insurmountable advantage in elections.
Into the picture comes Lyndon Johnson who though an @$$hole, was a very canny politician. He immediately realized the shift in power which would occur as a result of this initiative and so he launched his "Great Society" programs to bribe this newly created voting block into coming over to his side.
Over the course of about 10 years, Blacks migrated from the Republican camp into the Democrat camp, and have mostly remained there ever since. They have been effectively bribed with money from Washington D.C. to vote for Democrats.
Lyndon Johnson's strategy worked perfectly, though it has cost the nation about 21 trillion dollars in expenditures. It has however bolstered the Democrats power base for the last 50 years.
Most of the Liberal Republicans have since moved over to the Democrat Party, and now the big Cities of the North East are strictly Democrats. Indeed, most big cities everywhere are bastions of the Democrat party.
Good analysis, mostly.
But big cities have always been Democrats, going back well before the Civil War.
Antebellum Washington political power centered in the alliance of Southern Democrat slaveholders, whose votes were greatly enhanced by the Constitution's 3/5 rule, and Northern Big City Democrat financial/global merchant types whose votes were also greatly enhanced, by immigrants.
Today's Democrats are the same with the notable replacement of old slaveholders with the descendants of their former slaves.