It's simple: He's a Democrat in charge of a red state.
He won with only 49 percent of the vote in 2012 and faces a well-funded, well-organized opponent this time around. There haven't been any presidential polls in Montana this early, but four years ago, President Obama lost the state by 12 percentage points.
"Montana, all things being equal, has that Republican lean," said Jeremy Johnson, a political scientist at Carroll College in Helena. "Every Democrat has to run scared. No matter how strong a candidate is, you should treat it as the race of your life."
In Montana, says Johnson, even strong Democrats are at risk, and weak GOP candidates can potentially win. But this time around, Republicans believe they have a quality challenger against Bullock in Greg Gianforte, a wealthy businessman. Both Bullock and Gianforte are set to officially win their respective nominations in Tuesday's primaries.
"Gianforte is by far the best candidate from the Republican side that could have come out to challenge Bullock," said Shelby DeMars, a consultant with the Montana Group, a GOP consulting firm.
Gianforte made his fortune in software. Like many rich candidates before him, Gianforte suggests that because he doesn't have to solicit contributions from political action committees, he can't be bought. In addition, he argues that his business experience will help him build a stronger state economy. Although unemployment in Montana is low, the state perennially ranks near the bottom in terms of average salaries.
DeMars argues that people are "very tired of the status quo," however, many give Bullock good marks in terms of managing the state. Despite working with a Republican legislature, Bullock has gotten many of his proposals enacted into law, including a modest expansion of Medicaid.
"We got through the recession in pretty good order," said David Parker, a political scientist at Montana State University. "The fact that he's managed the state's expenses in a way so that there's money for a rainy day, that's a big deal -- particularly with the economic problems other states have had."
Bullock hasn't achieved everything he's wanted, though. One area of notable failure has been infrastructure. Bullock is now touting a $200 million plan, but his previous packages have been rejected by legislators.
"Voters can reasonably ask, you didn't get it done twice, what are you going to do differently?" said Parker.
Republicans are also seeking to make an issue out of Bullock having to reimburse the state for use of his official plane for campaign trips. They also note that the governor is on his third lieutenant governor, hinting at management problems.
In turn, Democrats have criticized Gianforte for filing a lawsuit several years ago seeking to block access to a river on his property. Access to public lands and waterways is a big deal in Montana.
Gianforte's campaign says the lawsuit is about a property misunderstanding, not public lands.
"The Gianfortes always have and always will be supporters of stream access," a spokesman told a reporter. "There's even a trail above the high-water mark waiting for you if you wanna come out and go fishing."
Nonetheless, Democrats believe they can use this issue to make Gianforte appear unsympathetic and out of touch. They have also been hammering him about donations that his family foundation has made to conservative and Christian groups, while accusing him of ducking questions about gay and transgender rights.
Some of these lines of attack may stick from either direction. But the election will likely come down to basics. Montanans may opt to re-elect an incumbent most believe has done a reasonably good job. But if voters' partisan inclinations prevail, Bullock could find himself looking for a new line of work.
*This story has been updated to include a clarification from the Gianforte campaign regarding its candidate's lawsuit about a river on his property.
