Jane Sanders, the wife of Senator Bernie Sanders of Vermont, marched with her husband this month through the Baltimore neighborhood of Freddie Gray, the man fatally injured in police custody, and took notes while they met with African-American pastors.

In Iowa, she schmoozed with supporters and reminded her husband to lighten his long, dark stump speeches. (“Doom and gloom!” she says she tells him. “Any hope at the end of the tunnel?”) She spent hours helping him prepare for this month’s debate, she reviews campaign ads, and come January, she will start venturing out alone as a surrogate for her husband in Iowa, New Hampshire and South Carolina.

Ms. Sanders may, with the exception of Bill Clinton, be the most politically active and experienced spouse in the 2016 presidential election.