Senate Democrats failed to roll back a Trump administration regulation that would expand the number of health insurance plans that don’t have to meet Obamacare’s benefit rules.

The Oct. 10 vote was 50-50 and needed a simple majority to pass. But Democrats used the vote to criticize Republicans’ health agenda just weeks ahead of the midterm elections. If it had passed the measure would have overturned a regulation expanding short-term limited-duration plans, insurance plans some deride as “junk insurance” and as an attack on Obamacare’s protections for people with pre-existing health conditions.

“In a few short weeks the American people...