The republican party was formed by whites in the 1850s as an instrument to end slavery in particular, and in general to reduce the size and scope of the federal government, which for a century had increased its powers in the name of enforcing and expanding slavery. The democratic party at the time existed primarily to preserve slavery and black codes. Its members constructed and liberally employed a derisive term, "black republican," to mean republicans who in addition to seeking the end of slavery, also sought full legal rights of citizenship for blacks. In cartoons, they depicted these republicans as racially black.
Lincoln officially claimed to not be a "black republican", but rather a republican who merely sought to end slavery by preventing its expansion into the new territories. Democrats insisted, though, that he was a black republican in white republican's clothing. Where they correct? Frederick Douglas initially thought not, but changed his mind after observing that within 4 years of taking office, Lincoln gave the most radical black republicans everything that they wanted: federal forces recruiting black troops and marching into slave states imposing emancipation and full black citizenship rights. Within 8 years of Lincoln's election (after the slavers made good on the bounty that they placed on his head when he began his 1860 office run), dozens of black Americans held state and federal office... as republicans. A revolution had taken place. A republican revolution.
With Lincon dead, though, democrats undid the Republican Reconstruction, and returned the democratic party to an instrument of imposing white supremacy. The democratic party in the south was the public face of the KKK.
Then came the 1960s, when a majority of republican US legislatures joined a *minority* of democrats to enact the civil rights laws that re-imposed the 1870s republican reconstruction. But since a democratic president introduced that legislation, the public credited democrats for it. And LBJ showed that white southern democrats could get black votes by supporting the legislation, using all manner of democrat principals of convoluted big government programs. Thus was born a 95% black voting block that existed for 30 years, from 1970 through 2000, a voting block so strong that its proportion far exceeded black support for various issues, including prayer in public schools, abortion rights, high "progressive" taxation, gay rights, school vouchers, and affirmative action (blacks support these democratic concepts at rates less than 95%, or in some cases oppose them).
The black voting block in the 2004 national election fell below 90%; in the crucial state of Ohio, it fell below 85%. And the republican party now stands to exceed the democrats in electing blacks to national office. Maybe ghetto parents will finally get school vouchers after all.