Which elections did Jim Crow laws "steal" for the Democratic nominee?

[Statilius the Epicurean]

Not at all. Read more carefully.

The 1948 convention barely passed its civil rights plank. The party as a whole wasn't exactly unambiguous on the point, and would have lost even more Southern EVs if it had been anything but.

Several decades of political history suggest that the subject of civil rights remained controversial within the Democratic Party for some time.

The point is that black voters in general and even Southern blacks in particular were crucial parts of both Truman and Kennedy's winning coalition, so it's completely ridiculous to argue that either candidate benefited from Jim Crow voter disenfranchisement.  

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In fact some people argue that black voters put Kennedy over the top in Missouri, Texas and South Carolina. (This thread has made me spend too much time researching African American voting patterns in the South.)

[Statilius the Epicurean]

Those are two different questions.

1) Would the Democratic Party have done better or worse in a political system where Jim Crow laws disenfranchising black voters in the South didn't exist?
2) Which specific Presidential elections did Democrats win directly because black people who that year supported the Republican candidate were prevented from voting? - Which was the question of the thread

1) is obviously unknowable, which is what you're now saying and I agree. But what you previously argued, that for 2) Democrats needed to keep black Southerners from voting in order to win in 1948 and 1960, is just historically incorrect when in fact black voters in the South helped carry both Truman and Kennedy to victory.