John Sullivan, a top U.S. State Department official and former Mayer Brown partner nominated to serve as the U.S. ambassador to Russia, appeared on Wednesday to disavow President Donald Trump's efforts to pressure Ukraine to investigate former Vice President Joe Biden, telling a Senate panel it would not be "in accord with our values" to ask a foreign government to probe a political rival.

Testifying at his confirmation hearing, Sullivan stopped short of directly denouncing Trump, saying he was unsure of whether the president had leveraged U.S. military aid as part of an alleged quid pro quo, which is now at the center of a House impeachment inquiry. Trump has denied he engaged in any unlawful or untoward conduct in his dealings with Ukraine.

Under questioning from Sen. Robert Menendez of New Jersey, a top Democrat on the Senate Foreign Relations, Sullivan, said the "concept of investigating a political rival … that would be inconsistent with our values."

"Soliciting investigations into a domestic political opponent—I don't think that would be in accord with our values," said Sullivan, a former co-leader of Mayer Brown's national security practice.

Sullivan's appearance before the Senate panel came as House investigators pressed forward with an impeachment inquiry centering on Trump's dealings with Ukraine.

In closed-door sessions, nearly a dozen State Department and other administration officials have testified about a July phone call in which Trump appeared to ask the president of Ukraine to investigate Biden and his son Hunter. The testimony has also addressed a shadow foreign policy overseen by Trump's personal lawyer Rudolph Giuliani, along with efforts to oust the former U.S. ambassador to Ukraine, Marie Yovanovitch.

Menendez asked Sullivan whether he knew that Giuliani was "seeking to smear" Yovanovitch.

"I believe he was, yes," Sullivan answered.

Later in the hearing, he added: "My knowledge in the spring and summer of this year about any involvement of Mr. Giuliani was in connection with a campaign against our ambassador to Ukraine."

Sullivan said he informed Yovanovitch, a career diplomat, of the Trump administration's decision to remove her from her post. When he inquired about why she was being removed, Sullivan said he was told only that the president had lost confidence in her. He pressed for further detail and received the same answer.

"I was told that he had lost confidence in her. Period," Sullivan said.

Sullivan noted that the president has broad authority to remove ambassadors, at one point acknowledging that his testimony could rankle Trump and prompt the White House to pull his nomination.

"He may decide he doesn't like my testimony today and doesn't want me to go to Russia," Sullivan said.

Mayer Brown Mayer Brown offices in Washington, D.C. (Photo: Mike Scarcella / ALM)

Sullivan joined the State Department in 2017 as deputy secretary of state following his departure from Mayer Brown. In financial disclosures, Sullivan revealed that he earned more than $650,000 in salary and bonus from the firm between 2016 and 2018. Sullivan's wife is a government investigations partner at King & Spalding in Washington.

Sullivan was nominated in October to replace Jon Huntsman, a former Republican governor of Utah, as the top diplomat in Moscow.

Early in his testimony, Sullivan said he did not organize a statement of support for Yovanovitch around the time of her removal, drawing an admonishment from Menendez. "I would've hoped that you would've spoken up a lot more loudly," Menendez said.

At a lighter moment in the hearing, Sen. Tim Kaine D-Virginia, raised the decisive seventh game in the World Series between the Washington Nationals and Houston Astros. Sullivan said that, while he's lived in Maryland for years, his baseball loyalties lie with his "Boston heritage."

"I just wanted to note that, until tonight, the pending World Series champions are the Boston Red Sox," he said.