The Indiana Democratic Party promised voters there is a choice for Senate “who will really lower your taxes.” But that choice wasn’t Democratic incumbent Sen. Joe Donnelly.

The mailer calls Libertarian Senate candidate Lucy Brenton “the anti-tax candidate” and says she would “cut red tape on small businesses, helping them create better jobs in Indiana.” It includes a side-by-side comparison of her to Republican Mike Braun, who, the mailer points out, is a former Democrat.

The Libertarian Party of Indiana posted a copy of the mailer on social media on Saturday, saying, “We wish to extend our gratitude to the Indiana Democratic Party for campaigning for Lucy Brenton for U.S. Senate.”

The mailer was sent to voters last week and said it was paid for by the Indiana Democratic Party, the Indianapolis Star reported.

Braun and Donnelly are locked in a fierce battle for the U.S. Senate seat, which Fox News has ranked a toss-up.

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“I am grateful for the free publicity highlighting my tax stance, but I think it will backfire,” Brenton told the Indianapolis Star. “Many Democrats are Constitution loving, fiscally conservative voters and my message resonates with them, too. Ultimately, how well they targeted the addresses will determine which voters learn that they have a choice in this race that demands all of their freedoms, all of the time.”

But the state Republican Party is chalking the mailer up to a “dirty trick” indicative of a “desperation move.”

“As we enter the final week before Election Day, it’s clear that Democrats are desperate to win, and they’ll do and say absolutely anything to beat Mike Braun – including trying to trick Hoosiers,” Kyle Hupfer, chairman of the Indiana GOP, said in a statement provided to Fox News.

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Earlier this month, Donnelly, who has served in the U.S. Senate since 2013, attempted to distance himself from the “radical left” and aligned his policies with President Trump and former President Ronald Reagan. In the campaign ad, Donnelly said he supported Trump’s proposed border wall and the U.S. Immigration and Customs Enforcement (ICE) agency – even as prominent progressive Democrats have called for it to be abolished.

Braun, too, has latched himself onto Trump during the campaign. A businessman and self-prescribed political outsider – despite serving as a state representative for three years – Braun has been endorsed by the president.

A spokesperson for the Indiana Democratic Party did not immediately respond to a request for comment from Fox News Wednesday.