President Trump escalated his campaign of retribution against his perceived impeachment enemies Tuesday, railing in the Oval Office about a decorated combat veteran who testified about the president’s conduct with Ukraine and suggesting the Defense Department should consider disciplining him.

“The military can handle him any way they want,” Trump said of Lt. Col. Alexander Vindman, who was ousted from his position on the National Security Council last Friday and reassigned to the Pentagon.

Asked whether he was recommending the military take disciplinary action against Vindman for his House testimony in the impeachment proceedings, Trump replied, “They’re going to certainly, I would imagine, take a look at that.”

Trump also leaped to the defense Tuesday of Roger Stone, his longtime former adviser and friend who faces a prison sentence after being convicted by a jury of obstructing Congress and witness tampering in connection with the Russia investigation.

Stoking new worries about improperly politicizing the Justice Department, Trump admonished federal prosecutors for recommending a seven- to nine-year sentence for Stone, which the president felt was too long.

Trump provided fresh evidence that he feels emboldened and will say and do as he pleases after the Republican-controlled Senate voted last week to acquit him in the impeachment trial.

Sen. Richard Blumenthal (D-Conn.) said Trump’s actions in recent days have seemed “almost delightedly vengeful” and are cause for “very deep and profound concern and alarm.”

“It completely explodes this delusion that he’s learned his lesson and he will turn over a new leaf, which was magical thinking from the start and a fig leaf for a number of my Republican colleagues,” Blumenthal said. “We ought to be very, very afraid of this kind of dictatorial personal vengeance against dedicated public servants who stepped forward to tell the truth.”

Republicans who control the Senate resigned themselves this week to the reality that they are unable to check or even influence Trump, even as some GOP strategists are warning that the president’s actions threaten the party’s Senate majority by complicating the home-state politics for a quintet of endangered incumbents.

One of them, Sen. Susan Collins (R-Maine), said last week that she believed Trump had learned “a pretty big lesson” by being impeached. But this week she said she had been so “concerned” about Trump’s desire to punish two impeachment witnesses, U.S. Ambassador to the European Union Gordon Sondland and Vindman, that she contacted the White House.

“My general position is that anyone who answers a congressional subpoena and comes forward and testifies should not face retaliation,” Collins said.

Asked what senators could do to rein in Trump, she replied, “I called to try to prevent the action.”

Yet the senator’s call did little to persuade the president. Trump last Friday fired Sondland and reassigned Vindman, as well as his twin brother, Yevgeny, also a lieutenant colonel, to the Defense Department. The Vindman brothers were escorted off the White House grounds, according to their lawyer.

Trump later took to Twitter to attack Alexander Vindman, who testified before the House last November that he was concerned about Trump’s conduct with Ukraine.

Some Senate Republicans said they were more concerned about Sondland’s fate. Sens. Ron Johnson (R-Wis.) and Thom Tillis (R-N.C.) said that they tried to save Sondland, a major GOP donor, from the embarrassment of being fired on the same day as Vindman. Both senators said they talked to senior White House officials when they learned that Sondland’s job was in jeopardy.

“I just wanted him to be able to leave with dignity,” Johnson said Monday evening. Sondland was said to have been willing to resign his E.U. post within a few weeks had he been given the time.

Neither Johnson nor Tillis raised any objection to the ouster of Vindman, a nonpartisan official who was awarded a Purple Heart for his service in the Iraq War.

Tillis said his motivation for contacting the White House was simply to create some separation between Vindman and Sondland’s dismissals as a way of helping to preserve the public image of Sondland, a Portland-based hotelier whose $1 million in donations to Trump’s 2017 inaugural committee paved his way into the president’s inner circle.

“I agreed with the decision on Vindman,” Tillis said. “I just felt like having the two have some distance would have been appropriate.”

Some Republican strategists are privately warning that Trump’s acts of retribution, should they continue, could endanger the reelection chances of five senators in competitive races: Joni Ernst of Iowa, Cory Gardner of Colorado and Martha McSally of Arizona, as well as Collins and Tillis.

“The White House and the [Trump] campaign need to look at this election through the lens of the vulnerable Republican senators,” said one top GOP strategist, who spoke on the condition of anonymity to avoid retribution from Trump or his aides. “It’s not that hard. You say to yourself before you make a move, ‘What’s this going to do to Cory or Joni or Martha or Thom or Susan?’ That’s what’s lacking here.”

Trump, the strategist continued, “is always going to do a little crazy every week because a little crazy is what motivates his base. But this is a team sport, and he does not want to lose the Senate. You saw what happened when he misjudged 2018 and lost the House. He got himself impeached. The Senate is the backstop.”

Democrats have zeroed in on Collins’s claim that Trump had learned a lesson from being impeached, an assessment she later clarified as a “hope” that he would not ask foreign leaders to conduct investigations against his rivals.

“She said that the president has learned his lesson. Those were her words, and then the next morning President Trump went out and disgraced himself before people of faith at the National Prayer Breakfast, and then President Trump turned around and held [a] make America hate again rally in the White House,” Rep. Hakeem Jeffries (D-N.Y.), one of the House impeachment managers, told reporters Tuesday.

Trump also has been seeking to retaliate against Sen. Mitt Romney of Utah, the lone Republican to vote to convict the president. In Tuesday’s Oval Office appearance, Trump slammed his longtime foil, telling reporters, “Romney’s a disgrace.”

Romney said Tuesday that he has received a fine reception from his Senate colleagues and in meetings with Utah GOP leaders following his conviction vote. Romney flew home to Salt Lake City last Thursday for meetings and then, signaling he wants to be seen as a team player, jetted to Palm Beach, Fla., to attend a big fundraising retreat for the National Republican Senatorial Committee.

Romney said in a brief interview following the weekly Republican senators’ luncheon, which was attended by Vice President Pence, that there was no awkwardness at the fundraiser or at Tuesday’s lunch over his rebellious vote.

“I’m sure people have different points of view; that’s what we’re entitled to do in a democracy,” Romney said.

Utah House Speaker Brad Wilson (R) described his visit with Romney as a “very frank” conversation, but one that GOP leaders appreciated. “It actually took a lot of courage to do that in the wake of what happened, primarily because many of us here are disappointed with what happened yesterday,” Wilson said Thursday.

Even as Trump has mocked him and his citation of his Mormon faith in guiding his vote, Romney said Tuesday that his Senate colleagues have been fully supportive.

“I certainly respect each of them for the conclusions they reached and believe that when people of character vote their conscience, that is a good thing,” he said.