The ad references the "Life at Conception Act," a so-called 2013 "personhood" measure Daines co-sponsored along with 129 others that defines life as beginning at fertilization of the egg. The point of the measure was to "implement equal protection for the right to life of each born and preborn human person," it read.
The measure also said, "nothing in this Act shall be construed to authorize the prosecution of any woman for the death of her unborn child."
Daines's position on abortion is that it should only be legal to save the life of the mother, the Billings Gazette reported in 2012.
Walsh's ad is backed by a six-figure buy for broadcast, cable and online placement, his campaign said.
Walsh and Daines are competing in a contest that is one of the most important in the battle for the Senate majority.
Daines's campaign declined to comment on the ad. The congressman is also going up with a deeply personal statewide spot featuring a woman. In the ad, Rebekah Uzenski of Bozeman details how her ex-husband would physically abuse her before she thanks Daines for supporting the Violence Against Women Act.
"I am so thankful there's men like Steve out there that actually stand up for women's rights and what is right," she says.