WASHINGTON (Reuters) - Democratic Senator and presidential hopeful Elizabeth Warren on Tuesday clashed with Mark Esper, President Donald Trump’s nominee for U.S. defense secretary, saying he should be not be confirmed to the post because of ethical concerns related to his former lobbyist job at defense contractor Raytheon Co.

FILE PHOTO - Democratic 2020 U.S. presidential candidate Sen. Elizabeth Warren speaks to members of the media during a town hall at the Peterborough Town House in Peterborough, New Hampshire, U.S., July 8, 2019. REUTERS/Elizabeth Frantz

The sharp exchange between Esper, the current Army secretary, and Warren was a rare contentious moment during his otherwise smooth Senate Armed Services Committee confirmation hearing, with panel members eager to fill the lingering Pentagon vacancy.

Esper refused when Warren demanded that he commit to not working for any defense contractor for four years after leaving his Pentagon job and that he extend a two-year ethics commitment due to expire in November that forces him to avoid decisions involving Raytheon.

He agreed to comply with all legally binding ethics regulations while in office. Warren, a candidate for her party’s 2020 presidential nomination and the only member of the committee to voice opposition to Esper’s confirmation, repeatedly interrupted him as he tried to explain his position.

There has been no confirmed defense secretary since Jim Mattis resigned in December over policy differences with the Republican president, the longest period in Pentagon history. Three people have served as acting secretary during that time, including Patrick Shanahan, who withdrew from consideration as Pentagon chief on June 18 after reports emerged of domestic violence in his family.

Numerous Cabinet members and senior White House officials have left the administration, either through firing or resignation, during Trump’s tumultuous presidency.

Esper, 55, is an Army veteran who has served as a congressional aide and as a Pentagon official under Republican President George W. Bush as well as a Raytheon lobbyist. He was introduced and given a strong endorsement at the hearing by Senator Tim Kaine, who served as 2016 Democratic presidential candidate Hillary Clinton’s vice presidential running mate.

While not criticizing Trump, Esper told the committee that he aligned himself with Mattis’ views on national security. Mattis, a retired four-star Marine Corps general, quit after Trump announced a plan to withdraw American troops from Syria, then pointedly mentioned in his resignation letter the need for the United States to treat allies with respect and be “clear-eyed about both malign actors and strategic competitors.”

Esper also said he would resign if asked to do something immoral, illegal or unethical.

Seeking to rebut Warren’s remarks, Esper also said he had never been accused of corruption in his life. Warren zeroed in on Esper’s seven years at Raytheon as vice president for government relations. She said that work raised questions about conflicts of interest.

“Secretary Esper, the American people deserve to know that you are making decisions in our country’s security interests, not in your own financial interests,” said Warren, who has prided herself as a check on corporate influence in the U.S. government.

“You can’t make those commitments to this committee. That means you should not be confirmed as secretary of defense.”

Speaking with reporters on Monday, Esper’s chief of staff Eric Chewning said, “I think the easiest way to think about the secretary’s (Esper’s) recusals is he is doing everything required by law. I mean, there’s no waivers, nothing outside of law, everything that he’s required to do by law.”

NO F-35 FOR TURKEY

Raytheon’s business is vast, and weapons sales involving the company could easily creep into policy discussions involving Esper. One example is Turkey. The United States warned Turkey against buying a Russian air defense system, instead pushing the NATO ally to buy America’s best alternative: Patriot missiles, made by Raytheon.

Chewning said on Monday that depending on the conversations, there could be instances in which Esper would need to recuse himself on the Patriot missiles issue.

Esper called Turkey’s acquisition of the Russian system “disappointing,” saying in a comment aimed at Turkey’s defense minister: “You can either have the S-400 or you can have the F-35 (fighter jet), you cannot have both.”

Warren slammed Esper for failing to commit not to seek a waiver from his existing government ethics agreement and cited concerns that the nominee is due deferred payments from Raytheon of at least $1 million after 2022. She also sought assurances that he would not quickly return to Raytheon or another defense firm after his Pentagon job.

Some panel members rallied to his defense. Referring to Warren, Republican Senator Rick Scott said: “I think she just needed a moment for her presidential campaign.”

Esper graduated from the U.S. Military Academy at West Point in 1986 alongside current Secretary of State Mike Pompeo. Esper also served in the 1991 Gulf War.

“I went to war for this country, I served overseas for this country, I have stepped down from jobs that paid me well more ... and each time it was to serve the public good,” Esper said in response to Warren.

“So no, I disagree with you.”