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.