As Trump campaign volunteers Rachel and Chris Gottberg prepared to knock on doors in York, Pennsylvania, last month, their goal was to win over the infrequent voters that the campaign sees as key to victory in the battleground state.

Wearing red T-shirts emblazoned with "Trump Force Captain," they were among a handful of door knockers who had gathered in the Republican Party headquarters in this working-class city of about 45,000. They planned to take along their 8-month-old baby in a stroller.

The couple said they were focused on newly registered voters and what political campaigns call "low propensity" voters — people who don't show up every voting cycle and may even skip the presidential ballot every four years.