Senate Minority Leader Chuck Schumer (D-NY) has successfully recruited prized political heavyweights in key battlegrounds, but left-wing insurgents are threatening to dash his plans to create a competitive midterm map for Democrats.

The toss-up Maine Senate race is perhaps ground zero for the ideological, and in some cases generational, clash, with embattled first-time candidate Graham Platner trouncing two-term Gov. Janet Mills (D-ME) by 34 points, 58% to 24%, in the latest polling for the Democratic primary to take on longtime centrist Sen. Susan Collins (R-ME).