This is from an op-ed written by Elijah Cummings himself for the Washington Post

I serve as chairman of the Oversight and Reform Committee, the primary investigative body in the House of Representatives. I have sent 12 letters to the White House on a half-dozen topics — some routine and some relating to our core national security interests. In response, the White House has refused to hand over any documents or produce any witnesses for interviews.

Let me underscore that point: The White House has not turned over a single piece of paper to our committee or made a single official available for testimony during the 116th Congress.

One of the most important investigations we are conducting is a review of White House security clearances. The White House argues that Congress is not entitled to any information about individual employees, including former national security adviser Michael Flynn, who pleaded guilty to lying about his communications with the Russians; current national security adviser John Bolton, who worked directly with the gun rights group founded by now-convicted Russian spy Maria Butina; or the president’s son-in-law, Jared Kushner, who was reportedly given access to our nation’s most sensitive secrets over the objections of then-White House Chief of Staff John F. Kelly and others.

Instead, the White House offered to let us read — but not keep — a few pages of policy documents that have nothing to do with the officials we are investigating, along with a general briefing on those policies during which they will answer no questions about specific employees.

We are also examining the president’s “hush money” payments to silence women alleging affairs before the election, as well as the president’s failure to divulge these payments on his financial disclosure forms, as required by federal law. Former Trump lawyer Michael Cohen provided copies of reimbursement checks signed long after Trump became president during our hearing with him, but the White House has refused to turn over any of the documents we requested. Instead, officials let us read 30 pages, about half of which were already public or entirely blacked out.

The White House has also refused to produce any documents or witnesses in response to our other investigations, including White House officials’ alleged use of personal email in violation of federal law; allegations that the president may have violated the Presidential Records Act by destroying documents; and reports from whistleblowers that the administration allegedly rushed to transfer sensitive nuclear technology to Saudi Arabia in violation of the Atomic Energy Act.

Interestingly enough it is not only the Democratically run Congress that the Trump White House is stonewalling, because they also recently got into a pissing match with Republican Senator Chuck Grassley:

Sen. Chuck Grassley is accusing the Trump administration of stonewalling him over a request for documents related to the Russia investigation, and he’s taking it out on the president’s nominee for a critical U.S. intelligence post.

The Iowa Republican’s demand for Justice Department documents on its probe of possible links between the Trump campaign and Moscow has left William Evanina’s nomination to head the National Counterintelligence and Security Center in limbo for the past year, frustrating the nation’s top intelligence leaders and even some fellow Senate Republicans.

Specifically, Grassley is asking for documents that he says House committees have received in their Russia probes, including those related to DOJ official Bruce Ohr, who became a conduit between former British spy Christopher Steele and the FBI. The senator recently placed a fresh hold on Evanina, who has been nominated twice and received overwhelming bipartisan support to head the agency that oversees government counterintelligence efforts.

It seems to me that the Trump Administration’s plan for dealing with these multiple investigations is to refuse to cooperate until they receive a subpoena and then fight that subpoena in court while claiming executive privilege. 

I am not sure how long they can hold out with that tactic, but it certainly speaks to the fact that they feel they have a lot to hide.