That isnt how politics work. One party can become very popular for a time (though democrats are FAR from popular right now), but then when things go bad, people want a change in leadership. This idea that democrats will win every election going forward simply isnt possible. If youre the only game in town, YOU will eventually be the bad guys and people will look for an alternative.
Instead of worrying about what republicans are going to do to win future elections, democrats should be working their asses off to keep voters on their side. I cant imagine that shutting down coal plants and stop oil drilling is going to work out very well for you guys.