Tester, a physically imposing figure who grew up on a family farm and lost three fingers in a meat grinder accident, enjoys a healthy approval rating in Montana despite his party affiliation. Fifty-six percent of registered voters in the state approve of the job he is doing, versus 33 percent who disapprove, according to a Morning Consult survey in April.
A rise in Tester's popularity with Montana voters came as Trump's standing in the state slid, according to separate Morning Consult polling. The president had 50 percent approval in the state in May, versus 47 percent disapproval. His approval rating fell from 56 percent in January 2017, according to the survey.
Those factors, combined with the perception that the GOP lost its best recruit for the seat when Ryan Zinke joined the Trump administration as Interior secretary, have contributed to nonpartisan election analysis sites pinning Tester as the favorite. Cook Political Report, Sabato's Crystal Ball and Inside Elections all list the race as leaning in Tester's favor, despite Montana's red tilt.
Tester held a 51 percent to 44 percent edge over Rosendale in June, according to a Gravis Marketing survey. The Rosendale campaign did not immediately respond to CNBC's request for comment.
The two-term senator's votes suggest a relative lack of fear about his re-election bid. Tester has voted with Trump's priorities about 37 percent of the time, less often than five colleagues considered the most endangered Democrats up for re-election this year: Joe Manchin of West Virginia, Heitkamp, Joe Donnelly of Indiana, Claire McCaskill of Missouri and Bill Nelson of Florida, according to FiveThirtyEight.
Tester opposed the confirmation of recent Trump nominees CIA Director Gina Haspel, Secretary of State Mike Pompeo and Health and Human Services Secretary Alex Azar. He voted against Justice Neil Gorsuch's confirmation to the Supreme Court last year. In February, Tester opposed a Trump-backed immigration proposal and a bill that would have withheld federal funding from so-called sanctuary cities.
The senator's immigration stance in particular has drawn Trump's ire. In April, he tweeted that "Democrats like Jon Tester continue to support the open borders agenda." The president argued that "we need lawmakers who put America First."
The campaign arm of Senate Republicans has attempted to lump Tester in with blue-state Democrats who oppose Trump's priorities. In a statement, National Republican Senatorial Committee spokesman Calvin Moore called the senator's Thursday ad "phony" and said Trump will hold Tester "accountable for constantly standing with radical leftists like Elizabeth Warren and Chuck Schumer to 'resist' any and all progress this administration is making in Washington."
Tester is certainly not Warren or Schumer, the Massachusetts and New York Democrats who have voted with Trump's priorities about 10 and 24 percent of time, respectively, according to FiveThirtyEight. For example, Tester joined Republicans in April in supporting a bill that would roll back some bank regulations. Warren was perhaps the fiercest Senate critic of the measure, while Schumer voted against it.
Tester seemed prepared for a Trump barrage as he helped to sink Jackson's VA secretary nomination in April.
"If he thinks it’s my job to sweep his stuff under the table and ignore our military folks, he’s wrong. If he thinks I should not be sticking up for veterans, he’s wrong,” Tester said at the time, according to Politico. “I look forward to working with President Trump. I’ve worked with him many times in the past, but we disagree.”