Anytime you have a chance to vote, take it, and vote for whatever option will be most likely to reduce the overall emissions. In most cases, this will probably mean voting for programs/parties etc that oppose conservatism.
I agree the the democrats are not nearly serious enough about stopping climate change, but the republican party is actively hindering efforts and definitely worse for the plant in the long run.
Neither party being serious does not mean one isn't way worse than the other
You still need to hold the Dems feet to the fire. Just handing them your vote as the least worst option just means you'll never be offered any good options. Make them work for it.
You can get involved in protests to try to make them pay attention. You can write letters to your representatives and prospective candidates. You can ask good questions at town halls. You can campaign for good candidates.
Voting is the least important thing you can do. Ultimately you have no choice but to vote for the least worst option but if that's the only thing you ever do, you're helping to bury all of us.
There is ideological diversity within the democratic party, especially at the local level. As part of a representative democracy, the electorate needs to be engaged with their representatives. You should start looking closely at your most local representatives.