During the negotiation of the Dominican Republic's annexation to the US, President Grant was fairly insistent that the Dominican Republic be made a state rather than a territory.
What would be the implications of the US having a mostly black/mulatto spanish-speaking state domestically?
Revolt and rebellion with the closest analogy I can make being the Philippines War several generations earlier. The Dominicans were fresh from the Restoration War against Spain, no one but then President Baez and his cronies wanted annexation, and Baez did not control much outside the capital. Annexation just means continuing the status quo revolt of a heavily militarized (i.e, 40% of men had military experience at some point in their lives) and hardened society while the vast bulk of the US army is stationed garrisoning the newly reconquered South.
Its just going to be a long, expensive, and unpopular guerrilla war with political implications. To secure this the US would have to do what the historical American occupation did (and this time on their own bill, rather than passing off to the Dominican treasury) and more: expand the army/navy and station several thousand troops, build a network of roads using imported or local labor (that has to be paid rather than corvee) to ensure troop movement in the countryside, build a new local government from scratch (good luck getting collaborators), expand the domestic industries to pay for all this, and increasingly give concessions to the middle and upper classes to maintain the barest minimum of support.
Historically in 1916-24 all this cost 10 million Dollars. Here I'd suspect it would cost more; the 1870s constituted what I can only call a warring states era of fiefdoms and caudillos rising and falling, with the poverty and ruggedness that'd imply.
But let me not be a spoilsport and try to answer your questions. Lets just assume there's enough will to finance an army expansion for a little bit, and the prospects of a strategic military base and profits from cash crops lead to better lobbying.
The implications of this lasting a state are pretty much a transition to an earlier sugar industrial economy funded by New York and New Orleans capital. Expect important numbers of businessmen to migrate south and try to register land, break up communal land, start a wage economy, and enforce positivism upon the population. This will not be popular at all and add to local grievances. This is probably going to have to be supplemented with immigration to get over the historical land shortages (Given the 1870s and 80s this will have to be Asian, Mexican, and European labor as Caribbean labor would be too expensive), and expanding large amounts of credit to the rancher and financier classes of the south and north respectively.
Sugar will soon give way to other prospectors, and so you'd soon get bananas, tobacco, cacao, mining, light industry, railroads, bankers, and so on. Things that need a ton of capital injection, are labor intensive, and generally lead to huge income inequality but thats everywhere in the time period so I can't say this is unique.
It'd be guaranteed electoral votes for the Republicans, two Republican Senators, and more Republican House Members. Who would be the state's Senators and Congressmen? How could they affect the policy and dialogue of national politics?
Initially a Baecista clique that quickly gets sidelined by the new, educated, and economically liberal sugar barons or mining magnates.
Sugar could actually make this state really rich in a short amount of time (alongside tobacco, spices, mining, and other industries) and Santo Domingo would start to compete with New Orleans in a few decades once the population takes off.
Part of the goal of annexing the DR was resettlement of Black Americans in the place (and this OTL happened a little bit in Samana bay). I would think that there'd be a certain amount of anglo-black movement to the state.
Very little honestly. In the immediate era of Reconstruction there were massive gains in the social position of freedmen, there's little incentive to move to a warzone with little official jobs. Later on the only people who could move to invest would be middle and upper class individuals that would have a stake in investing. Maybe then, a few decades after sugar has matured and reconstruction is rolled back that some waves of seasonal migrant workers come in to compete with immigrant labor. Depends on the cost and contracts.
There might be more of a fight over linguistic freedom in US politics. Towards the end of the 19th century there were some fights over German and Polish and Norweigan language schools in a number of states. I believe there were issues with the linguistic rights of Cajuns as well.
There would absolutely be more greater fights for linguistic freedoms. This state is not going to be swamped by Anglos, the entire elite would be universally multilingual, and I'm fairly confident the factors of US control would lead to a localist nationalism that would seek to preserve Spanish. That and honestly I think most Americans moving in, alongside immigrants would assimilate to the local culture rather than the other way around.
I don't know that it would stay Spanish-speaking.
Its staying Spanish speaking. There were over 400,000 Dominicans living in the country at the time, and new immigrants aggressively assimilated within a generation or two.
I Realistically, most black Americans aren't gong to get up and leave to go to whatever the state would be called.
I agree, there's little incentive when it seems things back home are improving until some sort of Great Migration happens.
Yes, black people and multiracial people comprise a majority of the island’s population together (at least, they do today - I do not know the demographic situation of this country in the 1870s), but whites of Spanish descent have been the economic and social elite of the island for centuries.
The elites of the Island were a lot more fluid than you think, and aside from the Gente de Primaria (the 200 or so old families that would trace their ancestry back to the early colonization) it was not uncommon to see people who Americans would consider nonwhite rise up the ranks and be mostly accepted. There never has been a strict planter/everyone else divide that you saw in the American South, and the concept of segregation was and still is anathema to the locals. The Republican Party, with its business focus and social liberalism represents the interests of far more individuals than the Democrats.