It's just what you do when your side doesn't have a justifiable platform on it's own merit: See: All the people who keep telling us to ignore all the bad stuff corporate dems do because Trump would be worse.
IF you could actually run on things people liked, you'd talk about that and perhaps only call out your opponent's opposition to the things you support or show how they might be lying about claims that they want similar things.
But when your core platform is "let rich people keep doing what they want," you have to find ways to deflect from that.
Conservatives do well because their ideology is compatible with the interests of capital. No party that is a serious challenge to those interests can win any notable power through elections in the US.
As far as the idea of focusing on local races: If your main concern is immediate and substantial action on climate, what good would winning a local race do for you? Yeah maybe it would be easier to get a left wing candidate on a school board or whatever, but that's because it holds no meaningful power.
Not that I think they have any particular chance of success at the national level. I've just found that "local races" argument... most charitably put, confusing, less charitably: bad faith or willfully missing the point.