What issue played a major role in the democratic party split

Civil Rights

Explanation: By the 1960s, the Democratic Party was made up of two distinct groups. One was the traditional base of the party, that is, white Southern conservatives. The other was progressives that made their mark under Roosevelt.

By the 1960s, the issue of Civil Rights had split the two groups as white Southern Conservatives were largely opposed to the issue of Civil Rights. Republicans, sensing this split, started to play towards the Southerners in what is now known as the "Southern Strategy."

The end result is that a split occurred and Southern Democrats left the party to become Republicans.

1. Congress leadership: Nehru and Gandhi underestimated Jinnah, Muslim League, its ambitions and outreach.
In the 1920s, Gandhi neglected Jinnah and appealed to Muslims for a common cause, which seemed harmless at that time.In the 1930s, Nehru, assured of the backing of his socialist ideology and the support among the Muslim bases for the same, arrogantly believed that Muslims won't be moved by a party based on faith. It later turned out he was false.In the 1930s, Jinnah was willing to negotiate an agreement with the leadership, but was constantly ignored.                                                This is going over my head. Arrogance and underestimation of someone doesn't go unpunished. However blame is to be equally shared by all the top leaders. In the 1940s, ML with almost a million members had no compulsion to bow out. He had no reason to cut a deal now. He was demanding recklessly. He refused to allow first independence , then partition. I may not comment on the course of history had this been allowed.While Gandhi tried his best possible to stop the painful separation, Nehru wasn't ready to the idea of Jinnah being the first Prime Minister of the independent India till the very end. Patel hoped partition would show Jinnah the fault in his ideas and would seal his fate.
Thus the failure of Congress leadership and the rise of Jinnah is interlinked.

2. Jinnah and the Muslim League: 
He instigated religious passions and fears among the masses since the 30s.He was more concerned with the fact that Muslims and Hindus didn't intermarry or interdine and that Congress didn't have Muslim representatives in the 1946 Provincial Elections. He claimed in his 1940 Muslim League Presidential Address that cultures, literature and way of living and views on life were different of the two communities. The theory of one nation had been carried along too far and that it was nothing but a far fetched dream. I don't think partition gave too much peace either.The provinvcial elections were held on the lines of religion and demand for a new country, rather than on the promises of growth and "independence".Direct Action Day was a call by this great leader. What else do you expect from such a gathering at such a crucial point of time on such a critical issue. If countries could be made by gatherings and processions, I'm going out on the streets to demand the Shubham Kingdom! The violence, retaliation and counter violence led to the inevitable- Partition of two brothers, fighting for petty issues, not ready to listen and even the parents acting as stubborn kids.

Hope it Helped :)