“Regardless of what you think about Senator Sanders’s politics, this is the single biggest thing that has happened to the insulin affordability crisis,” she said before the bus departed. Ms. Nystrom, who defined herself as a “conservative Democrat,” said she had not endorsed any presidential candidate but appreciated that Mr. Sanders had embraced this issue.

“We’ve never had this type of media coverage,” she said.

At 10:10 a.m., the bus rolls out of the parking lot in Detroit. A bag of snacks falls out of an overhead bin, prompting momentary panic from some reporters.

10:20 a.m. | The bus arrives at the first border checkpoint.

A United States Customs and Border Protection officer steps onto the bus and jokes about the well-functioning air-conditioning on a hot, humid day. The mood is very relaxed.

10:27 a.m. | Mr. Sanders attacks the drug industry.

Mr. Sanders has been holding court in the back of the bus, talking to patients. Not content to sit so far away from him, the journalists edge closer. The Vermont senator mostly asks the patients questions but also takes some time to rail against pharmaceutical companies, which he says are making “huge profits.”

[Here’s the latest polling and analysis of the Democratic primary race.]