A court ruling last November forced Pennsylvania to redraw its congressional districts in advance of the 2018 elections. So members like Republican Rep. Brian Fitzpatrick are left scrambling to win over thousands of new voters (in Fitzpatrick’s case, nearly 50,000 of them) in order to keep their seats.

Fitzpatrick isn’t the only one dealing with an unpredictable, unfamiliar electorate. Of the 16 races rated Toss-ups by Inside Elections with Nathan L. Gonzales as of Thursday, nine were in states where voters register as members of a political party, providing a window on changes in the voter population and alliances in those contests. All nine seats are being defended by Republicans.

On average, more than 7 percent of those registered as Republicans and Democrats in these districts are either newly registered, or moved to the district from elsewhere in the state in the past two years.

An additional 2 percent of the districts’ registered Republican and Democratic voters, on average, were members of another party in 2016. In these districts, which were already considered competitive based on previous election results, such a churn in the partisan makeup could make a big difference in the outcomes.

A mobile population and the tumult of the Trump era have also thrown wrenches into the traditional calculus of judging a district’s politics.

Watch: 12 Ratings Changes for House, Senate and Gubernatorial Races — 4 Toward GOP, 8 Toward Democrats