WASHINGTON - Hours after two nonpartisan campaign-finance watchdogs filed complaints yesterday with federal election officials and the US attorney general, a mysterious $1 million donor to a political action committee supporting presidential candidate Mitt Romney came forward and identified himself.

The contributor, Edward Conard, is a former executive with Bain Capital, which was cofounded by Romney, and said last night that he did not intend to circumvent election laws when he created a company, W Spann LLC, which paid the contribution without Conard’s name attached.

“I did so after consulting prominent legal counsel regarding the transaction, and based on my understanding that the contribution would comply with applicable laws,’’ he told Politico in a statement last night.

The Campaign Legal Center and Democracy 21, nonprofit organizations based in Washington, D.C., alleged in the complaints filed earlier yesterday that the contributor may have illegally sought to hide his identity by setting up a shell company, which formed and dissolved within weeks of making the contribution. Registration records for the company do not list its owners.

“This case deserves a good, hard look from the Federal Election Commission and the Department of Justice,’’ Paul S. Ryan, associate legal counsel for the Campaign Legal Center, said in an interview. “If violations are found, they should be prosecuted vigorously in order to deter future straw-contributor schemes that make a mockery of our campaign finance disclosure laws.’’

Federal law prohibits giving money in others’ names to political candidates or committees, and violators could face civil and criminal penalties for knowingly participating in such a scheme - either as the original contributor, the person or entity acting as a go-between, or the campaign or committee receiving the money.

The two groups also urged the commission and federal prosecutors to determine if W Spann should have registered as a political committee, which would require disclosure of the people behind it. Any entity whose major purpose is influencing elections and that spends more than $1,000 to do so must register with the commission.

“As far as we know the only thing we know this corporation has done is give money to a PAC,’’ said Fred Wertheimer, founder and president of Democracy 21 and - from 1981 to 1995 - president of Common Cause, a nonpartisan organization that has advocated campaign finance reform.

W Spann was formed by Boston lawyer Cameron Casey of Ropes & Gray and was registered in Delaware on March 15. On April 28, the company gave $1 million to Restore Our Future, a political action committee run by former Romney campaign advisers. On July 12, W Spann dissolved.