PORTAGE, Wis. — The attack ads began in early 2017, planting doubts well ahead of the 2018 midterm elections. Against ominous background music and storm clouds, the Republican-financed spots hit Tammy Baldwin, Wisconsin’s Democratic senator, over the Affordable Care Act, Iran and veterans’ health care.

By July, a Milwaukee radio station was carrying audacious ads about Ms. Baldwin’s support for abortion rights.

“Did you know one out of three babies aborted in America are black? One out of three. And Tammy Baldwin is a big reason why,” the ad said. “That could be the next Frederick Douglass or Rosa Parks or Martin Luther King they’re aborting.”

Then came the positive ads describing one of her opponents, Kevin Nicholson, as a former Marine; an “outsider”; a businessman; and, like Ronald Reagan, a convert to conservatism.