Republican U.S. Sen. Jeff Flake announced today that he will not seek re-election in 2018. While Flake was considered one of the most vulnerable incumbents on the ballot this cycle, his decision to retire may not improve Republicans' chances of holding the seat.

Part of what made Flake vulnerable was a primary challenge to his right from former state Senator Kelli Ward, who has gotten former White House aide Steve Bannon's endorsement. If Ward is the nominee, the GOP's fight to hold the seat becomes much more difficult, especially since Democrats have recruited a solid candidate in U.S. Rep. Kyrsten Sinema. Democrats would like nothing more than a Sinema-Ward general election.

Right now, there are more questions than answers. The most important is whether an open seat will attract a first-tier candidate who is acceptable to both establishment and Trump Republicans. Other candidates will undoubtedly get into the race now, but Republicans need one who brings some gravitas and fundraising ability to the contest. Today, the identity of that candidate (or candidates) is not obvious.

The contest will stay in the Toss Up column until the Republican field sorts itself out.