(CNN) Democrats are preparing for a major clash with the White House over getting Cabinet officials to testify before Congress, a sign of the fierce battles to come as they prepare for the most expansive probe of a sitting president in decades.

Democratic members of the House -- from its rank andfile to its leadership — have amped up their rhetorical attacks on Donald Trump's presidency, calling it "lawless" or likening it to an autocracy. Some new chairmen of powerful committees have threatened to compel Trump administration officials to come to Capitol Hill with subpoenas, while others are hopeful that they will voluntarily testify. And already some Democrats are saying that the executive branch is broadly resisting congressional oversight, pointing this week to two officials -- Health and Human Services Secretary Alex Azar and Treasury Secretary Steve Mnuchin -- who declined Democrats' invitations.

"It seems to be the pattern," House Ways and Means Chairman Richard Neal, a Massachusetts Democrat who has tried and failed so far to get Mnuchin to come before his panel, told CNN.

Their takeover of the House has given Democrats new powers to investigate the Trump administration and publicly question its officials about their policies and actions, including separating undocumented migrant families at the US southern border and calling thousands of Internal Revenue Service employees back to work without pay during the partial government shutdown. But in just a few weeks, Democrats have grown frustrated with Cabinet officials' efforts to delay or block their requests.

On Thursday, House Majority Leader Steny Hoyer, a Maryland Democrat, fumed at the Trump White House over Azar's and Mnuchin's decisions to not testify.

"It's what I expect from people who think they are authoritarian leaders of our government -- and it starts with the President of the United States," Hoyer told CNN.

The Democratic effort to bring in some of the Trump administration's higher profile officials is already well underway. After weeks of negotiations, acting Attorney General Matthew Whitaker has agreed to come before the House Judiciary committee on February 8. House Judiciary Chairman Jerry Nadler, a New York Democrat, recently wrote a letter to Whitaker detailing a number of questions he plans to ask him about the special counsel probe's into Russian interference in the 2016 election and the President's potential obstruction of it.

House Oversight Chairman Elijah Cummings, a Maryland Democrat, wants to know how the administration decided to add a question about citizenship to the census and whether Commerce Secretary Wilbur Ross misled Congress in his previous testimony on it. Even before taking the gavel in January, Cummings told CNN that Ross would be a key witness for his committee and, after weeks of negotiations, Ross agreed to testify before the panel on March 14.

Democrats are intent on getting a number of other Cabinet officials, including the secretaries of education and homeland security, to testify on Capitol Hill.

Rep. Bennie Thompson, a Mississippi Democrat who's the chairman of the Homeland Security Committee, told CNN that he had invited Homeland Security Secretary Kirstjen Nielsen to appear before his House panel -- and urged her to accept.

"There's a willingness to work with her on our part, but we won't hesitate if she chooses not to come to issue a subpoena," he said.

Earlier this week, Mnuchin declined to appear before the House Ways and Means Committee to testify on 36,000 Internal Revenue Service employees being called back to work without pay. On Thursday, Neal said it would be "premature" to subpoena Mnuchin and that he would instead send him a couple of more dates, seeming to want to start their relationship on the right foot.

"Traditionally a treasury secretary kicks off the Ways and Means Committee year," Neal said. "We want that to remain paramount."

In a letter to Neal released later that day, Mnuchin responded that he would come before the House Ways and Means Committee after the President's budget has been released. Rep. Kevin Brady of Texas, the top Republican on the Ways and Means Committee, said that Mnuchin "can handle himself in any setting -- and he's got a very positive story to tell."

Also this week, Energy and Commerce Chairman Frank Pallone personally requested that Azar testify before the committee, but the secretary's office declined the request.

"One way or another," Azar will have to testify before the panel, the New Jersey Democrat said in a statement.

He called Azar's denial of the request to testify "unacceptable" and said it is "outrageous" that the secretary has yet to appear before Congress for a hearing on separating children from their parents at the southern border. Between April 19 and May 31, 2018, more than 2,600 children were separated from their parents , after the Trump administration ordered federal prosecutors to pursue criminal charges against all migrants who crossed the border unlawfully. The policy was ultimately reversed with an executive order.

When they were in charge during the Obama administration, House Republicans spearheaded many investigations, including a high-profile, bitterly partisan one into the 2012 attacks on a US mission in Libya that brought then-Secretary of State Hillary Clinton to the Hill . On Thursday, they said that their Democratic colleagues should focus on opening the government.

"Why don't we have every committee focused on solving the shutdown problem? Then we can talk about what they want to do," House Minority Leader Kevin McCarthy, a California Republican, said Thursday.

"Ultimately the Democrats have to decide if they want to work to reopen the government to secure our border, to create jobs, or just to harass the President," Rep. Steve Scalise of Louisiana, the GOP whip, told CNN. "I fear the latter is their approach and I think it would be a mistake on their part."

Democrats said that the Trump administration officials' tactics would ultimately prove ineffective.

"Given that they will have to speak for the most corrupt, lawless and chaotic administration of our lifetime, you can hardly blame them for hiding under their sofas," Rep. Jamie Raskin, a Maryland Democrat on the House Judiciary and Oversight committees, told CNN. "But in American democracy, the government is held accountable through government officials. So they will have to testify."