A salty letter from the Treasury Department has prompted Sen. Ron Wyden (D-Ore.) to threaten to hold up President Donald Trump’s nominations to the department.

It all stems from the Democratic request for copies of Trump’s federal tax returns, which the department has blocked contrary to a federal law designed to grant lawmakers access to such documents.

Wyden had asked Treasury Secretary Steve Mnuchin on May 14 how many times the secretary has personally intervened in such requests, among other things.

Rather than answer the question, Treasury adviser Justin Sok responded by rehashing Mnuchin’s arguments about why the department shouldn’t comply with the initial request for Trump’s taxes from House Ways & Means Committee Chairman Richard Neal (D-Mass.).

“The reply I received was unresponsive and wholly unacceptable,” Wyden said in a statement.

“If the Treasury Department refuses to answer our questions, I am prepared to again place a hold on department nominees as I did previously when routine requests for information went unanswered,” he said.

Under Senate traditions, any senator can place a hold on a nominee to prevent or delay a confirmation vote, including as an effort to extract unrelated concessions from the executive branch. Thanks to a recent rule change by Senate Majority Leader Mitch McConnell (R-Ky.), however, which reduced the time it takes to cut off debate, holds may be less powerful than they used to be.

The Trump tax fight is one of the most prominent battles between the Trump administration and Congress. Mnuchin refuses to abide by the tax law requiring his department to hand requested tax returns to Congress, and he has defied a subpoena for those same documents. He claims that Congress has no legitimate legislative purpose for Trump’s tax returns ― a common claim being made by the Trump administration in its near-blanket stonewalling of congressional investigations.

In his letter, Sok reiterated Mnuchin’s main justification for not complying with the tax return request ― that the Justice Department’s Office of Legal Counsel is drafting an opinion letter indicating that asking for the president’s tax returns is somehow unconstitutional. “We will provide a copy of the opinion to you when we receive it,” Sok wrote.