As they say, if wishes were horses beggars would ride. Do you think there will in (say) the next 10 years enough "left-leaning Democrats" to pass single payer? Why and how should that happen?
Electoral power at the national level tends to swing back and forth between RP & DP on a 8-12 year cycle.
If you want single-payer, think not in terms of a Congress and President who want it but of what the conditions have to be for a President and Congresds who oppose it to nevertheless pass it. Most significantg 'progressive' legislation is passed under such conditions. The Congrress that passed the Civil Rights Act almost certainly didn't favor it but recognized that they had no choice.
Carrol