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South Carolina congressman Joe Cunningham addresses the South Carolina Democratic convention at the Columbia Metropolitan Convention Center on June 22. File/John A. Carlos II/Special to The Post and Courier

COLUMBIA — Eight months into his tenure in Congress, U.S. Rep. Joe Cunningham is drawing praise from an unexpected source: A conservative advocacy group funded by the Koch brothers.

Americans for Prosperity is paying for a digital and direct mail ad campaign thanking the Charleston Democrat for his vote against a budget deal in July that raised military and domestic spending by more than $300 billion.

"As negotiations continue on spending bills for the next fiscal year, we want lawmakers to know that we will have their backs when they stand on principle," said Americans for Prosperity Vice President Russ Latino. "We applaud their commitment to safeguarding the country’s economic future and hope that they will continue to stand strong for American families."

Founded in 2004, Americans for Prosperity helped to boost the tea party movement in 2010 and has generally supported Republicans over the years.

This latest ad campaign specifically thanked 26 lawmakers, seven of whom are Democrats, including Cunningham. The group declined to say how much they are spending on the ads.

In a statement on the day of the July vote, Cunningham cited the budget deal's impact on the national debt and deficit as the reason why he could not support it. He was one of just 16 Democrats to vote against the deal, which ultimately passed. Nearly 220 Democrats and 65 Republicans voted for the deal.

"Politicians on both sides of the aisle seem to have no problem with recklessly charging to the country credit card and leaving the tab for our children and grandchildren," Cunningham said. "Congress should come back to the negotiating table and deliver a bipartisan agreement that responsibly funds the government without further piling on to our skyrocketing national debt."

Cunningham's vote channeled the same fiscally conservative philosophy of his predecessor, former U.S. Rep. Mark Sanford, who lost his seat in a GOP primary last year. While campaigning around the historically Republican district, Cunningham repeatedly emphasized that he would be willing to work across the aisle.

Several Republicans have already filed to challenge Cunningham next year in what is expected to be one of the most competitive House races in the country for South Carolina's 1st Congressional District. The National Republican Congressional Committee has listed the district as one of its top 2020 targets to flip from blue to red.

S.C. GOP spokesman Drew McKissick dismissed Americans for Prosperity as a "D.C. special interest group."

"I'm sure those South Carolinians who donated to AFP will be disappointed that their money is going towards defending a liberal Democrat incumbent," McKissick said.

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Tyler Jones, a spokesman for Cunningham's reelection campaign, pointed to rankings that show Cunningham has been one of the more bipartisan members of Congress so far this year as evidence that "he’s not afraid to buck his own party when he thinks they’re wrong."

"Whether it’s voting against a reckless budget that added trillions to the debt or opposing a pay raise for members of Congress, Joe is walking the walk when it comes to fiscal responsibility and putting Lowcountry over Party," Jones said.

NRCC spokeswoman Camille Gallo called Cunningham a "fake engineer" — he has a degree in ocean engineering and was an engineer intern in Florida but does not have an engineering license — and noted that he votes with the "socialist agenda" of U.S. Rep. Alexandria Ocasio-Cortez, D-N.Y., 85 percent of the time, according to an analysis by the conservative Club for Growth.

"Enough said," Gallo said.

This is not the first time Cunningham has received appreciation from a group that typically backs Republicans. After the government shutdown earlier this year, the pro-business U.S. Chamber of Commerce cheered Cunningham and a few other Democrats for "working constructively with their Republican counterparts to bring government employees back to work."

Follow Jamie Lovegrove on Twitter @jslovegrove.

Jamie Lovegrove is a political reporter covering the South Carolina Statehouse, congressional delegation and campaigns. He previously covered Texas politics in Washington for The Dallas Morning News and in Austin for the Texas Tribune.