Basically: anti-gerrymandering rules in several Democratic-leaning states man that there are a whole lot of competitive seats in the house of representatives. Vote, especially downballot, and even if you think you’re in a place that’s “safe” for one presidential candidate or another.
The anti-gerrymandering laws in Democratic-leaning states largely passed before the 2010 redistricting when Republicans swung the gerrymander heavily in their favor.