It would fracture the party. It would alienate moderates. How would they attract white voters in November? They'd lose!
And second, Truman won the election in November.
Caro's assessment is that in winning the fight for a civil rights plank, Humphrey did more than anybody else to help Truman get elected.
I think this focus makes sense. No outcome is more important this year than defeating Trump (though taking the Senate is a close #2).
I think that's the right thing to do. And I also think that's how we win.