(CNN) House Democrats, eager to investigate all aspects of the Trump White House, have identified former administration officials they need to carry out their probes. But the White House isn't making it an easy task.

On at least two occasions, the White House has pushed back on efforts by the House Oversight Committee to reach out directly to former officials, chiding the committee's powerful Chairman Elijah Cummings for not contacting the White House first. Their contention: Topics pertaining to former officials' work should be sorted out with the White House first -- even if the officials are no longer federal employees.

At the center of one of the disputes is former chief of staff John Kelly, whose cooperation is central to Cummings' investigation into the White House's handling of security clearances, including Trump's son-in-law and senior adviser Jared Kushner. Cummings' staff reached out to Kelly directly by phone on multiple occasions, which prompted a stinging response from White House counsel Pat Cipollone.

"Once again, these actions disregard my earlier request to the Committee regarding contacts with former or current White House officials," Cipollone wrote to Cummings in a late February letter, adding it's "vital" such outreach is first done through the White House counsel's office. "Consulting with my office will ensure that the Committee efficiently obtains access to the information and individuals to which it is entitled and that any disclosure of privileged information to Congress is properly authorized."

Cummings told CNN that the move is tantamount to "stonewalling," pointing to examples from past administrations of former federal officials being asked to testify directly before Congress.

"The President's lawyers they know that's not accurate. It's not accurate," Cummings said. "So they're basically stonewalling That's what it is."

Asked if he had made any progress in getting cooperation from Kelly as he left the Capitol on Thursday, Cummings said, "Not yet."

"They block us from everything."

The move is part of a larger White House strategy to make it difficult for Democrats to succeed in their aggressive investigations into the President -- investigations the White House views as rampant overreach and an improper effort to obtain privileged communications with the President.

Though publicly maintaining they will cooperate with all requests, White House officials have internally taken an aggressive approach to the oversight requests. Part of the White House's strategy has included slow-walking responses to demands for hearing dates and delaying efforts to turn over documents or schedule transcribed interviews.

Democratic officials say there have been at least 30 examples of the administration resisting demands for documents in just the first three months of the Democratic-controlled House. On Friday, the Trump administration missed a Democratic deadline to provide documents on Trump's interactions with Russian President Vladimir Putin.

Cipollone holds regular meetings in the West Wing to devise the administration's response to individual requests. These meetings, which are typically run by Cipollone's deputy Mike Purpura, are often limited in attendance because the White House is so prone to leaks. The meetings include few, if any, communications staffers, which has caused friction between the legal team and public-facing officials, who have protested that they aren't being kept in the loop so don't know what to tell the media.

In his briefings with the President, Cipollone has not held back about how expansive the House Democratic requests are, sources told CNN. In the words of one official, "he's not sugarcoating it."

A source familiar with the process said Cipollone has taken a more aggressive approach toward congressional requests than his predecessor Don McGahn. That strategy is evident in his sternly worded letters to Cummings.

"You haven't seen Don McGahn sign letters like that," the source said.

But this change in strategy could be because McGahn felt less of an obligation to respond to Democratic lawmakers when they were in the minority in the last Congress, a separate person added.

Though Purpura is leading the White House's response to requests from the Hill, sources said Cipollone has been engaged, working the phones to discuss oversight requests with those affected and even sitting down with Cummings, as he acknowledged in a recent letter.

Despite the aggressive approach, a senior White House official defended how the administration has handled the Democratic requests so far, arguing that the White House's response is measured compared to Cummings' approach of circumventing them to interview former officials.

On a call with the House Oversight Committee earlier this month, aides from the committee said that the White House refused three times to confirm or deny that a memo from Kelly on White House security clearances even existed.

House Republicans say many of the Democratic requests are out-of-bounds, particularly when it comes to Cummings' request for details on the security clearance process in the White House -- and concerns Kelly and McGahn privately voiced about Kushner and Ivanka Trump's security clearances.

"I'm comfortable with the commander-in-chief having sole authority of who gets clearance and who doesn't -- I'm totally comfortable with that," said Rep. Jim Jordan, the top Republican on the Oversight Committee when asked if the White House should comply with Cummings' request on security clearances. "I think the President of the United States has the absolute authority on who gets clearance and who doesn't."

Cummings also has been battling with the White House over his investigation into whether a Trump attorney, Sheri Dillon, and a former White House official, Stefan Passantino, misled federal ethics officials about the President's initial failure to disclose payments he made as part of a hush-money scheme to silence stories about his alleged extramarital affair with adult-film actress Stormy Daniels.

Passantino no longer works at the White House and is now an attorney at the Trump Organization. But in a letter to Cummings, Cipollone said that the committee was making "grossly unfair" allegations against both Passantino and Dillon -- and said that Passantino would not sit down for an interview.

"In response to your request, given longstanding law and practice, we are not inclined to make the former Deputy Counsel to the President available for a transcribed interview inquiring into his conversations and advice he provided while serving as Deputy Counsel to the President," Cipollone wrote, referring to Passantino.

Asked earlier this week if he would subpoena Passantino and Dillon, Cummings said: "I haven't gotten there yet."

But Cummings contended that "there's precedent" for House panels to reach out to former officials without consulting with the White House first. A Democratic aide provided CNN with letters from 2013 when then-GOP Chairman Darrell Issa reached out to three federal officials -- former Environmental Protection Agency Administrator Lisa Jackson, former Energy Department official Jonathan Silver and former White House deputy technology chief Andrew McLaughlin -- at their office addresses.

The fight between Cummings and the White House counsel's office could set the tone for the broader Democratic House as it conducts numerous investigations of the Trump administration and Trump's businesses.

For instance, House Intelligence Chairman Adam Schiff has said he is likely to bring back some of the officials that were interviewed in the Republican-led Russia investigation from the last Congress who refused to answer questions about their time in the White House, such as Steve Bannon.

Many of the same officials Cummings has reached out to as part of his probe on security clearances have also been contacted by House Judiciary Chairman Jerry Nadler, who sent letters to 81 individuals and entities earlier this month as the opening salvo in his investigation into possible corruption, obstruction of justice and abuse of power. Nadler said Thursday that about half of the recipients indicated they would provide documents by Monday's deadline, while a handful have said they would not do so.

"We've heard from a number of other people who have said they will comply if we give them a subpoena -- they want an excuse, they want a friendly subpoena, so-called," Nadler said. "There are a couple of people who have been defiant and said they're not going to comply, but that's only a handful."

This story has been updated with additional developments Friday.