While the idea you propose is a popular one among a portion of the liberal population, I think it's edges toward fantasy. It's based on the flawed idea that opposition toward President Obama is based primarily on his half-African ancestry, and not primarily on his political philosophy and executive policies.
Are the 29% of Hispanics who supported Trump "racist?" And what about the people who voted
twice for Obama, who in the last election supported Trump, are they also "racist?"
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To the OP, in a hypothetical scenario, Trump's economic and immigration policies result in a general increase in the American economy and personal economy prosperity. With a increase in local tax income (and the typical belief on politicians that the money will never end) liberal city governments increase spending on social programs leading to the creation of sanctuary districts.
Housing, food, jobs, medical, initially the districts work reasonable well and the idea spreads. Great as long as employment numbers never fluctuate and the move to increased automation never happens.
In the last year of Trump's second term of office a number of factors (domestic and international) drive a slight downturn in the economy, for the most part it's a minor hiccup, but the hiccup results in increase in unemployment and homelessness by tens of thousands.
Fearing a reduction of tourism, business and lifestyle, local governments make their districts all but mandatory for all homeless. Tough anti-vagrancy and anti-panhandling laws passed by city councils give big city mayors the power to have the police congregate "street people" into the districts.
One year later Sisko, Dax and Bashir arrive in San Fransisco.