FILE PHOTO: Senator Ron Wyden (D-OR) arrives for a Senate Intelligence Committee hearing evaluating the Intelligence Community Assessment on "Russian Activities and Intentions in Recent US Elections" on Capitol Hill in Washington, U.S., May 16, 2018. REUTERS/Joshua Roberts/File Photo

WASHINGTON (Reuters) - The top Democrat on the U.S. Senate Finance Committee on Tuesday called for Congress to investigate whether President Donald Trump’s appointees are trying to hide political interference in the Internal Revenue Service over releasing his tax returns.

The demand from Senator Ron Wyden comes as the White House attempts to block efforts by the Democratic-led House of Representatives to obtain tax returns and other financial information from Trump and his companies.

Treasury Secretary Steven Mnuchin last month refused to comply with a congressional subpoena seeking six years of Trump’s tax returns, saying there was a not a “legitimate legislative purpose” for the request.

However, an internal IRS legal memorandum said the Treasury secretary did not have the authority to turn down such a request, according to a May 21 report in The Washington Post.

Wyden sent a letter to IRS Commissioner Charles Rettig citing the Post, with questions about the memo.

“Congress needs to investigate to know whether Trump appointees have engaged in a cover up to conceal political interference inside the IRS,” Wyden wrote. “Career IRS officials prepared a legal analysis stating that the agency was required to provide the returns, and it was buried. We need to know who saw this memo and when.”

As a member of the minority party in the Senate, Wyden cannot launch a congressional investigation, but can pressure the Republican Senate committee chairman in various ways.