San Francisco’s irrepressible former mayor, Willie Brown, was walking along one of the city’s streets when he happened to run into another former city official that he knew, James McCray.

McCray’s greeting to him was “You’re ten.”

“What are you talking about?” Brown asked.

McCray replied, “I just walked from Civic Center to Third Street and you’re only the tenth black person I’ve seen.”

That is hardly surprising. The black population of San Francisco is less than half of what it was in 1970, and it fell another 19 percent in the past decade.

A few years ago, I had a similar experience in one of the other communities further down the San Francisco peninsula. As I was bicycling down the street, I saw a black man waiting at a bus stop. As I approached him, he said, “You’re the first black man I have seen around here in months!”

“It will be months before you see another one,” I replied, and we both laughed.

Actually, it was no laughing matter. Blacks are being forced out of San Francisco — and out of other communities on the San Francisco peninsula — by high housing prices.

At one time, housing prices in San Francisco were much like housing prices elsewhere in the country. But the building restrictions — and outright bans — resulting from the political crusades of environmentalist zealots sent housing prices skyrocketing in San Francisco, San Jose, and most of the communities in between. Housing prices in these communities soared to about three times the national average.

The black population in three adjacent counties on the San Francisco peninsula is just under 3 percent of the total population in the 39 communities in those counties.

It so happens that these are counties where voters and the officials they elect are virtually all liberal Democrats. You might be hard pressed to find similarly one-sided conservative Republican communities where blacks are such small percentages of the population.

Certainly that would be hard to find in states with a substantial total population of blacks. In California, a substantial black population has simply been forced by economics to vacate many communities near the coast and move farther inland, where the environmental zealots are not yet as strong politically, and where housing prices are therefore not yet as unaffordable.

With all the Republican politicians’ laments about how overwhelmingly blacks vote for Democrats, I have yet to hear a Republican politician publicly point out the harm to blacks from such Democratic policies as severe housing restrictions, resulting from catering to environmental extremists.

If the Republicans did point out such things as building restrictions that make it hard for most blacks to afford housing, even in places where they once lived, they would have the Democrats at a complete disadvantage.

It would be impossible for the Democrats to deny the facts, not only in coastal California but in similar affluent strongholds of liberal Democrats around the country. Moreover, environmental zealots are such an important part of the Democrats’ constituencies that Democratic politicians could not change their policies.

Although Republicans have a strong case, none of that matters when they don’t make the it. The same is true of the effects of minimum-wage laws on the high rate of unemployment among black youths. Again, the facts are undeniable, and the Democrats cannot change their policy, because they are beholden to labor unions that advocate higher minimum wages.

Yet another area in which Democrats are boxed in politically is their making job protection for members of teacher unions more important than improving education for students. No one loses more from this policy than blacks. For many of them, education is their only chance for economic advancement.

But none of this matters so long as Republicans who want the black vote think they have to devise earmarked benefits for blacks, instead of explaining how Republicans’ general principles, applied to all Americans, can do more for blacks than the Democrats’ welfare-state approach.

— Thomas Sowell is a senior fellow at the Hoover Institution.