In our system of government, congressional oversight of the intelligence community is critically important -- and not optional. Intelligence agencies and professionals have a responsibility to cooperate with lawmakers on the intelligence committees and make available materials they request.

At least, that's the way it's supposed to work. House Intelligence Committee Chairman Adam Schiff (D-Calif.) told ABC News over the weekend that the National Security Agency has begun "withholding" information, including materials that may be directly relevant to Donald Trump's Ukraine scandal.

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"That is deeply concerning," the Democratic leader said. "And there are signs that the CIA may be on the same tragic course. We are counting on the intelligence community not only to speak truth to power, but to resist pressure from the administration to withhold information from Congress because the administration fears that they incriminate them."

Yesterday, in a CBS News interview, he went a little further.

"We have requested intelligence, relevant intelligence, concerning Ukraine as a part of our oversight responsibility," Schiff said when asked what evidence his committee is requesting. "The intelligence agencies -- some of them have stopped cooperating. And it's our understanding they're doing this on the instructions of others, or with the advice of others." Schiff said information about Ukraine's reaction to the pressure campaign by Trump allies in 2019 "would be very pertinent" and should be handed over to investigators.

The Intelligence Committee chairman added, "If there was more evidence that bears on that question and it's being withheld by the intelligence community at the urging of the president, that is a corruption of the intelligence community."

It's worth emphasizing that administration officials have denied Schiff's allegations. That said, the track record of Trump administration denials isn't great.

The Ukraine scandal is obviously already serious, but if Trump or members of his team directed the intelligence community to hide materials from Congress, the controversy may yet intensify.

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