[corrral]

Yep. Currently reads like they're going to propose... something (marriage?) "in August ban on lawmaker stock trading". Problem is that phrases like "August ban" are common in headlines, but that's not what's intended here—it's not saying "the ban that happened/will-happen in August", as in "alleged mob boss nabbed in August sting operation", but describing when they'll propose a ban.

A fix could indeed involve commas:

House Democrats to propose, in August, ban on lawmaker stock trading