But when did black Americans become so heavily Democratic?
For that, we turn to data compiled by the Joint Center for Political and Economic Studies. The Joint Center pulled data from independent research, Gallup polling, exit polls, professional polling firms and their own surveys to put together a look at the partisan makeup of black voters since the administration of Franklin Roosevelt. That the data start in 1936 and not, say, with the emancipation of slaves after the Civil War -- thanks largely to a Republican president -- is because the ability of black Americans to vote was regularly restricted and uneven.
In the decade before 1948, black Americans identified as Democrats about as often as they did Republicans. In 1948, as Real Clear Politics' Jay Cost wrote a few years ago, Democrat Harry Truman made an explicit appeal for new civil rights measures from Congress, including voter protections, a federal ban on lynching and bolstering existing civil rights laws. That year, the number of blacks identifying as Democrats increased.
The second big jump is the one that you likely thought of first: The Civil Rights Act of 1964. Its passage in July of that year was the culmination of a long political struggle that played out on Capitol Hill. When he signed the bill, President Lyndon Johnson reportedly said that Democrats would, as a result, lose the South for a generation. It's been longer than that.
It's important to note, though, that African Americans were already voting more heavily for Democrats than Republicans. At no point from 1936 on, according to Joint Center data, has the Republican candidate for president gotten more than 40 percent of the black vote.
Compared to the overall vote, the extent to which black voters have backed Democratic candidates has grown -- a little in 1948 and 1952, but a lot beginning in 1968.
It's worth adding another line to that graph: The Democratic vote in the heart of the South, including Alabama, Georgia, Louisiana, Mississippi and South Carolina. The average support for the Democratic candidate each year has slipped downward, but plummeted in 1948 and 1964. In the latter year, those states backed Barry Goldwater. In the former, they largely backed the States Rights party candidate, Strom Thurmond.
The gap in partisan support among black voters predates World War II. But the yawning chasm seen in the chart at the top of this post can be traced back to two seminal moments of civil rights advocacy.
The shift in Southern voting toward the Republicans can be, too.