(CBS/AP) NBC newsman Tim Russert testified Wednesday he never discussed a CIA operative with vice presidential aide I. Lewis "Scooter" Libby, contradicting Libby's version to a grand jury in the CIA leak investigation.

The testimony came as prosecutors prepared to rest their perjury case against Vice President Dick Cheney's former chief of staff.

The courtroom testimony so far has provided a rare view inside a White House under fire during the lead-up to the war in Iraq, reports CBS News correspondent Gloria Borger. It's not pretty: top advisers squabbling in the West Wing, leaked secrets and faulty memories — all part of a full-blown damage control operation apparently led by the vice president himself.

Russert, the host of "Meet the Press," testified about a July 2003 phone call in which Libby complained about a colleague's coverage. Libby has said that, at the end of the call, Russert brought up war critic Joseph Wilson and mentioned that Wilson's wife worked for the CIA.

"That would be impossible," Russert testified Wednesday. "I didn't know who that person was until several days later."

That discrepancy is at the heart of Libby's perjury and obstruction trial. He is accused of lying to investigators about his conversations with reporters regarding Wilson's wife, CIA operative Valerie Plame.

During Libby's 2004 grand jury testimony, he said Russert told him "all the reporters know" that Wilson's wife worked at the CIA. Libby now acknowledges he had learned about Plame a month earlier from Cheney but says he had forgotten about it and learned it again from Russert as if new.

Libby subsequently repeated the information about Plame to other journalists, always with the caveat that he had heard it from reporters, he has said. Prosecutors say Libby concocted the Russert conversation to shield him from prosecution for revealing information from government sources.

Plame's identity was leaked shortly after her husband began accusing the Bush administration of doctoring prewar intelligence on Iraq. The controversy over the faulty intelligence was a major story in mid-2003.

Given that news climate, defense attorney Theodore Wells was skeptical about Russert's account.

"You have the chief of staff of the vice president of the United States on the telephone, and you don't ask him one question about it?" Wells asked. He followed up moments later with, "As a newsperson who's known for being aggressive and going after the facts, you wouldn't have asked him about the biggest stories in the world that week?"

"What happened is exactly what I told you," Russert replied.

Russert originally told the FBI that he couldn't rule out discussing Wilson with Libby but had no recollection of it, according to an FBI report Wells read in court. Russert said Wednesday he did not believe he said that.

Wells spent a considerable amount of time in cross-examination Wednesday undermining Russert's general recollection of past events, reports CBSNews.com's Jennifer Hoar.

Wells seized upon a 2004 interview that Russert had granted to a Washington Post reporter in which he failed to mention a phone call he had made to a different reporter, from Russert's hometown paper, The Buffalo News. Russert was later lambasted for not remembering the call in an article titled, "Tim, Don't You Remember?"

Russert was also grilled about whether or not he had told the NBC News President Neal Shapiro that he had discussed his 2003 phone call with Libby with an FBI agent. Russert said he didn't recall if he had mentioned that to Shapiro, Hoar reports. Wells appeared troubled by this as Russert indicated that he and Shapiro were close.

Russert will be back on the stand on Thursday; Wells says he has about two more hours with his cross-examination.

Special Prosecutor Patrick Fitzgerald has spent weeks making the case that Libby was preoccupied with discrediting Wilson. Several former White House, CIA and State Department officials testified that Libby discussed Plame with them — all before the Russert conversation.

Fitzgerald has said Russert would be his final witness. Prosecutors spent the past few days playing audiotapes of Libby's grand jury testimony in court. In the final hours of those tapes Wednesday, Libby described a tense mood in the White House as the leak investigation began.

Though President Bush was publicly stating that nobody in the White House was involved in the leak, Libby knew that he himself had spoken to several reporters about Plame. He said he did not bring that up with Bush and was uncertain whether he discussed it with Cheney.

Libby did remember one conversation with Cheney, however, in which the vice president seemed surprised when told by his aide where Libby had learned Plame's identity.

"From me?" Cheney asked, tilting his head, Libby recalled.

Libby said he had forgotten that Cheney was his original source until finding his own handwritten notes on the conversation. The notes predated the Russert phone call by more than a month.

Vice President Cheney is likely to take the stand in Libby's defense, adds Borger. But he may need to answer some questions about himself and whether he directly controlled a White House effort to discredit a political enemy.