A federal appeals court decision that could strike down the Affordable Care Act as soon as this month has rattled officials in several states who are pursuing legislation to preserve some coverage in the absence of any Trump administration contingency plan.

Lawmakers in states including Louisiana, Nevada, New Mexico and California have passed bills or are reviewing action aimed at dealing with the fallout if the ACA is overturned. Many of these lawmakers are also facing re-election campaigns this fall in some of the very states...