WASHINGTON (AP) — An ethics watchdog group filed a federal complaint Thursday accusing Republican Sen. David Vitter of accepting illegal campaign contributions from a California dry cleaning company that wanted stimulus money.

The group Citizens for Responsibility and Ethics in Washington called the donations a ‘‘conduit contribution scheme’’ used by a private company to curry favor with the Louisiana lawmaker facing re-election this year. It asked the Federal Election Commission to investigate and penalize Vitter and the company.

Questions have swirled around a fundraiser Vitter held with executives of U.S. Dry Cleaning Corp., since The Times-Picayune reported recently that company officials and their spouses gave him nearly $40,000 last year.

It remains unclear why they gave to Vitter. The company has no clear ties to Louisiana, and the officials were not regular political donors.

Also, one of the four company donors told the newspaper that U.S. Dry Cleaning reimbursed his $4,800 contribution. Such reimbursements are illegal, in part because companies are barred from donating to federal candidates. The donor added that the company was seeking Vitter’s help getting stimulus financing or Small Business Administration assistance, although Vitter strongly opposed the stimulus bill.

Vitter campaign spokesman Luke Bolar said the senator never made any request on behalf of the company but didn’t respond to questions about why the company officials contributed to his campaign, or what issues they discussed. In a statement, he said Vitter welcomes an investigation and believes the donors should be prosecuted if they violated campaign finance laws.

U.S. Dry Cleaning has not responded to requests for comment.

A spokeswoman for the FEC said the agency has received the Vitter complaint and will investigate if it is deemed valid. In similar cases, the government has assessed fines and at times pursued criminal charges against people for disguising the true source of campaign contributions.