Democrats used to focus on climate change as one of their big-ticket issues to discuss. Now this has changed as democrats focus less and less on the issues with the climate in their campaigns and actions. In 2000, Al Gore ran his campaign on the concept of climate change and what he would do as president to try to prevent it from occurring. While he didn't win the election, his ideas on climate change became part of the Democratic Party's values.
Recently, Democrats haven't been bringing up climate change as much, downplaying the significance of the issues impacting our planet in an attempt to appeal to moderate voters.