House Intelligence Chairman Adam Schiff (D-Calif.) said on ABC's "This Week" that he wants former White House national security adviser John Bolton to testify in the Ukraine investigation, but that he assumes the Trump administration will fight House investigators in court to prevent him from appearing.

"John Bolton is a very important witness. We already know from the testimony of others that this is someone who was concerned that people in the State Department — Ambassador Sondland and others, Mulvaney — were cooking up a "drug deal." And by that he meant a corrupt deal, involving withholding a White House meeting or perhaps the withholding of aid as well, for these political purposes."

— Schiff

The big picture: Trump's former Russia adviser Fiona Hill testified that Bolton told her to alert the chief National Security Council lawyer that Rudy Giuliani was "cooking up" a "rogue operation" with acting White House Chief of Staff Mick Mulvaney and Gordon Sondland, the ambassador to the EU.

The Washington Post reported on Thursday that Bolton warned U.S. Trade Representative Robert Lighthizer that Trump would likely oppose restoring some of Ukraine's trade privileges — an indication that the suspension of assistance to Ukraine may have extended beyond just military aid.

Bolton is currently in talks to testify in the impeachment investigation, the New York Times reports.

What to watch: Schiff also told ABC's Martha Raddatz that he expects the committees to start conducting public hearings "soon," but wouldn't outline a specific timeline.

Go deeper: John Bolton to write Trump book, pairs with top agents