WASHINGTON—With a vote on his impeachment looming next week, Defense Secretary Pete Hegseth is digging in his heels and vowing to stay at his Pentagon post for the rest of the Trump administration.

Hegseth has reassured his allies that he is not angling for Marsha Blackburn’s Tennessee seat in a Senate that will now be controlled by Democrats, Pentagon officials said.

If his congressional opponents vote to impeach him, Hegseth will seek to use the confrontation as a platform to aggressively defend his agenda, which includes the war with Iran, the ouster of senior military officers and rolling back past diversity policies.

“He’s not going anywhere,” said Tim Parlatore, a special adviser to Hegseth. “He intends to serve the full four years.”The embattled defense secretary’s defiant stance has set the stage for a bruising clash with lawmakers frustrated over the stalemated conflict with Tehran. Even Republican lawmakers who have backed Trump on the Iran war are voicing serious misgivings about Hegseth.

“I can tell you that there’s a lot of questions about Hegseth,” Rep. James Comer, the Kentucky Republican who chairs the House Oversight Committee, said in an appearance on NewsNation. “We have no idea what the strategy is moving forward on Iran.”While more lawmakers would like to see Hegseth depart the Pentagon, his place within the Trump administration appeared to be more secure late Friday when Trump announced that Stephen Miller would be his next chief of staff.

Hegseth has long aligned himself with Miller, favoring the expanded use of the military to secure the southern border and strike suspected drug smugglers.

Within minutes of Miller’s appointment, Hegseth posted “Patriots in control” on his personal social-media account and shared a photo of himself with the newly appointed White House chief of staff.

Midterm election setbacks have sometimes been followed by shake-ups on the White House’s national security team. After Democrats won control of the Congress in 2006 during a low point in the U.S. war in Iraq, President George W. Bush acknowledged the “thumping” by Democrats and announced the resignation of Defense Secretary Donald Rumsfeld.

Trump has told associates privately, and said publicly, that he is still backing Hegseth. But Hegseth’s critics hope that Trump will reconsider that support if congressional opposition to the defense secretary begins to imperil the Pentagon’s spending requests, including for military operations in the Middle East.

Hegseth has qualities Trump has long prized, including unqualified loyalty to the president, which the defense secretary has conspicuously displayed in speeches, television appearances and social media.

In a video taken on the steps of the Pentagon, Hegseth portrayed himself as a warrior for Trump’s agenda, suggesting that his ouster would be a rebuke to the White House and the broader MAGA movement.

“We are more determined than ever, more resolute than ever to finish what we have started,” Hegseth said in the video released Thursday. “We will stand. We will fight, and we will defend this republic from all enemies, foreign and domestic.”Charles Stevenson, a lecturer at Johns Hopkins University who wrote a history of defense secretaries, said the social-media campaign Hegseth and his aides have launched to save the Pentagon chief’s job was without precedent and could backfire in Congress.

Hegseth’s supporters at the Pentagon have described the push for impeachment as political grandstanding that has little chance of succeeding. A two-thirds vote by the Senate would be required to convict and remove Hegseth from office.

Joel Valdez, the Pentagon’s deputy press secretary, said Hegseth’s team had convened this week to map out an “aggressive defense” of the defense secretary. “What we’ve learned from this president is never back down,” Valdez told reporters.

But retiring Sen. Thom Tillis (R., N.C.) warned in an interview that a defiant performance from Hegseth during a Senate impeachment trial would further damage the administration’s relationships on Capitol Hill and torpedo the Trump administration’s hopes of securing additional military funding.

“He has poisoned the well so much with members of Congress,” including many Republicans, Tillis said of Hegseth. “He should also pay attention to what happened on [Election Day] Tuesday because it wasn’t a ringing endorsement of him doing the same thing of ‘fight, fight, fight.’ ” Tillis provided the pivotal vote to confirm Hegseth last January, but said he now has a “profound case of buyer’s remorse” and wouldn’t only support impeachment but would “encourage others to do the same.”The senator predicted that Hegseth would at minimum get a vote of no confidence from more than half the Senate, even if there aren’t sufficient votes to remove him.

Rep. Adam Smith, the Washington-state Democrat who is expected to helm the House Armed Services Committee next year, said he supports the effort to impeach Hegseth.

“Hegseth thought he was like the head of some cheerleader pep rally, as opposed to running an organization, and fired a lot of very intelligent people,” Smith said in an interview. “There’s plenty to work with.”Even if Hegseth survives an impeachment this year, Democrats could impeach him again next year when they gain control of Congress and might have as many as nine more Democratic senators.

The new Democratic majorities also will wield subpoena power in both chambers, with the ability to seek documents and depositions under oath from Hegseth and other Pentagon officials. Using the power of the purse, and a handful of Republican votes in the Senate, they could also try to deprive the Defense Department of the money required to continue the Iran war.

One way out of the Trump administration’s dilemma, some Republicans have speculated, might be for Blackburn, the newly elected governor of Tennessee, to appoint Hegseth to her vacant Senate seat. But a Tennessee GOP operative close to her said she and Trump haven’t spoken about it, and that Hegseth isn’t being considered.

“This is nothing more than Washington, D.C., spin from a bunch of consultants with nothing better to do,” the operative said. “The Blackburn team is focused on the transition, not entertaining baseless speculation. D.C. won’t pick the next senator.”