We are not there yet though: intermittence (and to some extent construction speed) still favor nuclear power but maybe not for that long.
If you asked the typical person who's extremely concerned about climate change where we should invest money, they'll answer wind and solar far more likely than nuclear. There are people who advocate for nuclear as the solution, sure, but they're in the minority. Often they're people who don't seem especially concerned about climate change.
There is no such Party, and no such Party Platform; the document is, as its own title says, the Democratic Platform 2016.
> does not mention nuclear energy once.
It promotes it without naming it, by proposing pricing in the climate externalities of fossil fuels, which makes all energy sources that don't contribute to warming, including nuclear, more competitive in the market and attractive to private investment.