CONCORD, N.H. — The troubles of a for-profit college company have found their way into a tight U.S. Senate race.

Republican U.S. Sen. Kelly Ayotte is coming under fire from her opponent for accepting donations from Bridgepoint Education, which being investigated by the U.S. Department of Justice.

Ayotte, who is facing Democratic Gov. Maggie Hassan in one of the country’s most competitive Senate races, last month returned the $8,000 in donations from Bridgepoint after she learned of the company’s problems. She received four donations over two years.

Hassan’s campaign has stepped up efforts this week to highlight the donations and suggested Ayotte tried to cover them up, something Ayotte has denied.

Hassan spokesman Aaron Jacobs, in questioning Ayotte’s behavior, referenced fraud allegations against presumptive Republican presidential nominee Donald Trump’s failed real estate school, Trump University.

“This new federal investigation into Bridgepoint Education is just the latest in a growing number of state and federal probes into this shady Trump-style university connected to Kelly Ayotte, and it’s time for Ayotte to come clean about why she told Bridgepoint that she’d continue to stand up for their shared beliefs even as Bridgepoint engaged in predatory practices,” Jacobs said.

Democrats also say the donations imply Ayotte is beholden to special interests, noting she sent a thank you note to the company for the money.

Ayotte is one of dozens of Republicans and Democrats, according to the Center for Responsive Politics, who received donations from Bridgepoint in the 2016 election cycle.

Ayotte’s campaign dismissed the allegations, noting the donations were returned June 30, and said a thank you note was a standard practice for campaigns.

“This is a false charge being pushed by Governor Hassan,” Ayotte campaign spokeswoman Liz Johnson said Thursday. “Our campaign returned these contributions as soon as we knew about the problems with this group.”

Ayotte’s campaign pointed out Hassan has returned donations, too. During Hassan’s 2014 gubernatorial run, she returned tens of thousands of dollars to a labor union political action committee after the attorney general’s office ruled the donation broke campaign finance laws. And, this year, she made donations to domestic violence advocacy groups after it was revealed she accepted a donation from a former teacher accused of sexual misconduct.

Bridgepoint Education Inc., which is based in San Diego and runs Ashford University and the University of the Rockies, did not respond to a request for comment.