And just as importantly, she had expressed serious interest in running. Others had not.
In the aftermath of the Hagan news—first reported by Roll Call and confirmed by National Journal—Democratic strategists in Washington suggested they had a long list of viable alternatives the party could turn to.
"There are a lot of Democrats who can win this seat," said one strategist with ties to the Democratic Senatorial Campaign Committee. "Anthony Foxx, Janet Cowell, Tom Ross, Heath Shuler, Josh Stein are all more than formidable."
But it's not that simple. Many of them are seen, at least within North Carolina, as unlikely to run for Senate. Transportation Secretary Anthony Foxx, for instance, is considered by many party strategists to be the party's strongest possible nominee. The African-American former mayor of Charlotte has the kind of star power to drive liberal turnout. But Foxx has announced publicly, and told Burr personally, that he won't run for Senate. Josh Stein, another rising star, is considered highly unlikely to abandon his run for attorney general.
Of all the possible alternatives, state Treasurer Janet Cowell is likely to receive the most attention in the coming weeks. A combination of business background and liberal bona fides have made her popular across the Democratic Party, and she could draw support from EMILY's List's stable of national donors as a woman who supports abortion rights.
But in April, Cowell announced she would seek reelection to her current position, essentially taking herself out of consideration for the Senate. Many Democratic officials in the state are skeptical she'd be interested in a federal campaign.
There's also Ross, the former University of North Carolina president, and Shuler, a former congressman. Neither has ruled out a Senate run—but neither has publicly communicated interest in the race, either, nor do they currently hold elected office.
Democrats are hopeful they can reprise recent Senate history and convince one of these prospects to jump in after all. Cory Gardner, arguably the GOP's best candidate in 2014, initially told Republicans he wouldn't run. In 2007, Hagan herself passed on a bid before changing her mind.
Democrats hope that with a heavyweight like Hagan out of the picture, candidates who once said no could suddenly rethink their futures. Some of them might even have higher ceilings than Hagan, whose campaign experience came with high unfavorable ratings.
"If Kay had run, she would have started with higher name ID and higher negatives than Burr, which is not something normally seen from a challenger," said one senior Democratic strategist.
Democrats can also take solace in the fact that North Carolina was never likely to make or break a Senate majority next year. Wisconsin, Illinois, New Hampshire, Florida, and a handful of other states are all better bets for Democrats to gain the four seats they would need if they hold the presidency.
But without a top candidate in a purple state like North Carolina, Democrats might not be able challenge Burr at all, and Hagan's decision not to run may have all but erased one of the Democrats' target states from the 2016 map.