No one really gave Cornell College professor Dave Loebsack a chance of knocking off Republican Rep. Jim Leach in 2006. But the Democrat won that race, and more than a dozen years later, he’s announced that his current seventh term in Congress will be his last.

Democrats now have to defend a competitive open seat that wasn’t previously on the list of vulnerable districts.

The 2nd District is the most Democratic of Iowa’s four seats, but it still had a GOP lean to it recently. President Barack Obama carried the 2nd by 13 points in 2012, while Donald Trump carried it by 4 points four years later. Loebsack had a close call in his 2010 re-election bid (when he won by less than a point), but he’s never dipped below a 5 points in his victories since. In 2014, Republican Joni Ernst won the 2nd by nearly 2 points in her winning Senate race against Democrat Bruce Braley.

With the seat now open, we’re changing our Inside Elections rating of the race from Solid Democratic to Toss-up. That pushes the total number of vulnerable Democratic seats to 40, compared to 29 for the GOP. Republicans need a net gain of 18 or 19 seats for a majority, depending on the outcome of the new election in North Carolina later this year.

Obviously, the candidate fields for the open seat are only beginning to take shape.

On the Democratic side, state Sen. Zach Wahls and former state Sen. Rita Hart were two of the first names mentioned. Hart, who was also the party’s nominee for lieutenant governor last year, was considering a run for the Senate against Ernst, but may now choose to run for this open House seat instead. On the Republican side, potential candidates include Fort Madison Mayor Brad Randolph and state Sen. Chris Cournoyer.

Democrats could have a second open House seat to defend in Iowa if freshman Rep. Cindy Axne decides to challenge Ernst. Axne’s 3rd District is about 7 points more Republican than the 2nd. And her race is currently rated as a Toss-up but would be even more vulnerable if it’s an open seat.

With competitive races for president, Senate and all four House districts, Iowa will be a prime battleground in 2020.

Also watch: What race ratings really mean and how we create them