Who could be an effective democrat candidate to take on trump in 2020.?
I'd vote for any candidate that actually cares about people more than their position and agenda. Sadly, most politicians these days have gotten really good at preaching that they care for people, while pushing policies that are harmful to people in the long run.
We may end up with the horrible situation that we went through in 2016 with two terrible candidates. However, this time around, the benefit of the doubt will be on the side of the democrat. I can't imagine Trump winning again, if he is even still legally able to run or even if the Republicans would nominate him again. Lots of stuff can happen in two years. My point is that "effective" is a relative thing, and it won't take much to be effective against Trump.
"Effective democrat" is an oxymoron.