As the sitting governor, Bullock has already won two hard-fought elections, in 2012 and 2016, and he had a strong approval rating in the few available polls. Bullock is also starting with very high name recognition and should be able to raise ample money, making him a far more imposing challenger than the existing field of little-known Democratic contenders. Indeed, businesswoman Cora Neumann had raised the most money and was likely the Democratic frontrunner before Monday, but she quickly bowed out of the race and endorsed Bullock.

Bullock's decision to run provides Democrats with their best shot at ousting Daines, but it will still be an uphill battle. Montana backed Trump by a punishing 56-35 in 2016, and there's little indication, at least yet, that Trump will fare significantly worse there in November. Furthermore, Daines has done little to endanger himself beyond any generic Republican incumbent.

In an era in which fewer and fewer voters are willing to split their tickets, especially in federal races, Bullock will have a tougher time convincing Trump-leaning voters to cross over for him when the stakes appear higher. However, there's still a real chance he could pull off an upset win, and his running expands for Democrats what has long been a narrow path to flipping the Senate.