The political network backed by billionaires Charles and David Koch is launching a $4 million advertising campaign against two Senate Democrats facing re-election in 2018, as it starts to fulfill a pledge to spend $20 million selling the federal tax overhaul passed late last year.

The television and online ads are critical of Sen. Joe Donnelly of Indiana and Sen. Claire McCaskill of Missouri for voting against the legislation. They're two of the 10 Senate Democrats on the ballot in states won by President Donald Trump in 2016 and potentially vulnerable to Republican challengers.

The Koch-affiliated group Americans for Prosperity is sponsoring the ads, which are scheduled to start Thursday and run for about three weeks.