The Missouri branch of Americans for Prosperity says it's spending $1.8 million on ads against Sen. Claire McCaskill.
The television and digital ad announced Wednesday by the Koch-backed group accuses the Democratic incumbent of saying she supports tax cuts but voting against President Donald Trump's tax bill.
A narrator in the ad says Missourians can spot a "politician who says one thing but does another."
McCaskill repeatedly said she supports tax cuts for the middle class. But McCaskill says she voted against the law because it gave preferential treatment to the rich.
A press release from McCaskill's campaign responding to the ad called the law "partisan" and said it was a "massive giveaway to pharmaceutical companies and billionaires."
McCaskill is among several Democrats up for re-election in states Trump won.