By Rebecca Ballhaus
WASHINGTON -- A group backed by billionaire industrialists Charles and David Koch on Thursday launched a $4.5 million advertising campaign urging three vulnerable Democratic senators to support the Republican effort to overhaul the U.S. tax code.
Americans for Prosperity, one of the Koch network's largest political advocacy groups, will air ads in Missouri, Wisconsin and Indiana encouraging those states' Democratic senators -- Claire McCaskill, Tammy Baldwin and Joe Donnelly, respectively -- to back the tax proposal unveiled by President Donald Trump and GOP leaders last week.
An Americans for Prosperity official said the ads mark a "new phase" for the group's push for tax reform, in which it plans to directly target lawmakers. Sens. McCaskill, Baldwin and Donnelly are up for re-election in 2018 and are seen as vulnerable in states Mr. Trump won by substantial margins in the 2016 election.
Mr. Trump already has personally targeted Mr. Donnelly, who joined the president last week in Indianapolis for his unveiling of the tax plan. In that speech, Mr. Trump thanked the senator for joining him but warned him to support the plan. "If Sen. Donnelly doesn't approve it, because you know he's on the other side, we will come here," Mr. Trump said. "We will campaign against him like you wouldn't believe."
The ad campaign by Americans for Prosperity, a nonprofit, marks one of the most substantial financial investments into Republicans' latest push to overhaul the tax code.
The proposal sketches out a range of tax changes -- including lower taxes on corporate profits, incentives for business investment, fewer and lower individual income tax brackets and the end of estate taxes -- that Republicans say will boost economic growth and benefit middle-income families. Administration officials in recent days have pushed back against criticism that the plan would provide its biggest benefits to the wealthy.
Other conservative groups also have launched major ad campaigns on tax reform in recent months. American Action Network, a group linked to House Speaker Paul Ryan (R., Wis.) in August launched a $500,000 digital ad campaign and said it expected to spend $20 million on the effort this fall. The U.S. Chamber of Commerce launched a seven-figure ad campaign last month, targeting Republican members of the House who have been shaky on their support for tax reform.
Opponents of the tax proposal also have opened their wallets in recent months, with the progressive group Not One Penny launching a series of ad campaigns targeting Republican lawmakers.
The Koch network, which didn't support Mr. Trump during the 2016 election, has sought to bolster its ties to the administration in recent months. Americans for Prosperity President Tim Phillips last week attended a dinner at the White House with the president. Next week, Vice President Mike Pence is set to deliver the keynote speech at a retreat in New York City hosted by the network for its top donors.
Write to Rebecca Ballhaus at Rebecca.Ballhaus@wsj.com