I set my sights for the Memorial Day recess on Mr. Tester, one of 10 Senate Democrats up for re-election this fall in states President Trump won. In Washington, Mr. Tester has been doing things differently from the rest of that pack, voting often with liberal colleagues and, in one well-publicized episode, helping to take down Mr. Trump’s nominee to lead the Department of Veterans Affairs.

I wanted to see why Mr. Tester felt so confident flouting the president. I wanted to talk to Montanans who have sent him twice to Washington despite, in overwhelming numbers, calling themselves Republicans. And I wanted to see where Mr. Tester, the Senate’s only active farmer, returns each weekend. Covering Congress, it is easy to fall into a trap of seeing politicians and the votes they cast strictly as products of their political parties. Following them home reminds you that politics actually come from somewhere.

The Tester farm is outside the town of Big Sandy, Mont. Mr. Tester grew up on this farm. His grandparents homesteaded it a century ago. The home-styled butcher shop where he lost three fingers as a child is just a few yards away. He told me that when he built his current house on the property, he oriented it at an angle so that the views would capture the limitless prairie encircling it, rather than a picturesque set of mountain peaks in the distance. The prairie, he said, was what he’d come from and how he’d made his living.

In Washington, I cover veterans policy, meaning I spend a lot of time following Mr. Tester and his Republican counterpart, Senator Johnny Isakson, through the Capitol. But here was something totally different: Mr. Tester hammering away inside the elbow of a broken grain auger, climbing skyward and then balancing himself atop a grain storage bin, bantering with his wife and farming partner, Sharla.

“Is your hand in there?” Ms. Tester yelled out at one point, when her husband — who has already lost three fingers — shoved his arm into the grain auger. “You’re nuts,” she sighed when he told her to fire up the machine anyway. (He came out fine.) Here was a man, it quickly became clear, who on the farm and in Washington trusts his instincts enough that he is willing to take risks others will not. Many Montanans, he believes, prefer it that way: They may not agree on everything he does, but they know where he stands and believes that he comes by it honestly.