Appearing on Fox News Sunday, Mulvaney responded to host Bill Hemmer's question about whether the request by House Ways and Means Chairman Richard Neal, D-Mass., to view Trump's tax returns would be successful, saying the issue had already been settled.
"Oh no, never -- nor should they," Mulvaney said. "That's an issue that was already litigated during the election. Voters knew the president could have given his tax returns. They knew that he didn't and they elected him anyway."
Neal on Wednesday said he submitted his request to IRS Commissioner Charles Retting, seeking six years of Trump's personal tax returns, in addition to some of his business entities.
Trump responded to news of the request Wednesday saying it seems he is "always under audit" but "until such a time as I'm not under audit" he would not be inclined to release his tax returns.
William S. Consovoy, a lawyer for the president, said in a letter to Treasury Department General Counsel Brent McIntosh on Friday that Trump has the right to keep his tax returns private and Neal's request "flouts" the "fundamental constitutional restraints," which protect privacy and First Amendment rights, adding it could set a "dangerous precedent."
Mulvaney agreed with Consovoy's assessment Sunday, saying that a fundamental purpose of tax law is to protect tax filers' privacy and that Democrats know they won't get the returns and "just want attention on the issue because they don't want to talk to us about policy."
"If they don't get what they want in the Mueller report, they're going to ask for the taxes," said Mulvaney. "If they don't get what they want in the taxes they're going to ask for something else. It doesn't surprise anybody."
Rep. Sean Casten, D-Ill., responded to Mulvaney's appearance on Twitter Sunday, saying Congress has the right to see Trump's returns and that it is more important to retain "the trust of the electorate" than to protect the president's financial privacy.
"Our democracy increasingly depends on you and your colleagues understanding that," wrote Casten, who is not a member of Neal's committee.