Already fighting from behind, Democrats' chances of keeping the House have slipped further in the last month.
In a letter to Democratic colleagues on Thursday, House Speaker Nancy Pelosi called on House members to make a point of defending their record on crime.
Republican groups including Citizens for Sanity, headed by former aides to Trump, poured money in recent weeks into ads criticizing Democrats as weak on crime and illegal immigration.
In a Reuters/Ipsos poll released Tuesday, respondents were twice as likely to list crime, rather than abortion rights, as the country's biggest problem.
Shield PAC, a Third Way-affiliated political action committee, launched ads on Monday promoting the law enforcement records of Virginia's Elaine Luria and Minnesota's Angie Craig, both endangered Democrats.