The newly sworn-in Republican congressman George Santos, whose campaign résumé has been shown to be largely made-up, is the subject of a complaint filed with the Federal Election Commission.

The complaint concerning the New York representative was filed with the FEC on Monday by the Campaign Legal Center (CLC), a non-partisan watchdog group.

Santos won his seat, which covers parts of Long Island and Queens, in November.

He has since been the subject of relentless scrutiny, exposing claims about his education, business career and family background, including claims to be descended from Holocaust survivors and that his mother’s death was a result of the 9/11 attacks.

Santos has admitted “embellishing” his CV. He is under investigation by authorities in New York and in Brazil, in the latter case over alleged use of a stolen chequebook.

His Democratic predecessor in the New York seat has called him a “conman” and members of Congress have called for action against him.

But Republican leaders have taken no action and after last week’s five-day standoff over Kevin McCarthy’s bid for speaker, Santos – who cast one vote for McCarthy while appearing to make a white supremacist sign – is now a member of the US House of Representatives.

In a statement on Monday, the CLC alleged that Santos and his 2022 campaign committee, Devolder-Santos for Congress, “violated federal campaign finance laws by engaging in a straw donor scheme to knowingly and willfully conceal the true sources of $705,000 that Santos purported to loan to his campaign”.

The group also said Santos “deliberately report[ed] false disbursement figures on FEC disclosure reports, among many other reporting violations; and illegally us[ed] campaign funds to pay for personal expenses, including rent on a house that Santos lived in during the campaign”.

The complaint notes multiple campaign expenditures, widely reported, of $199.99, one cent below the $200 FEC threshold for the provision of receipts.

It also notes that Santos has struggled to explain the source of his wealth, and says it is “likely” that after losing an initial run for Congress in 2020, he “and other unknown persons worked out a scheme to surreptitiously – and illegally – funnel money into his 2022 campaign.

“The concealed true source behind $705,000 in contributions to Santos’s campaign could be a corporation or foreign national – both of which are categorically barred from contributing to federal candidates – or one or more individuals, who would be precluded from contributing such a large amount, far in excess of [official] contribution limits.”

Citing reporting by outlets including the New York Times, the CLC complaint says: “Particularly in light of Santos’s mountain of lies about his life and qualifications for office, the [FEC] should thoroughly investigate what appear to be equally brazen lies about how his campaign raised and spent money.”

Santos did not comment, CBS News reporting that he declined several requests.

Adav Noti, senior CLC vice-president and legal director, told the same network: “Voters deserve the truth. They have a right to know who is spending to influence their vote and their government and they have a right to know how the candidates competing for their vote are spending those funds.

“George Santos has lied to voters about a lot of things, but while lying about your background might not be illegal, deceiving voters about your campaign’s funding and spending is a serious violation of federal law.”