(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.