WASHINGTON — Former President Donald Trump on Thursday intensified his attacks on Democratic lawmakers, calling for their arrest and suggesting severe punishment after a group of legislators released a video urging members of the military to disregard unlawful orders — a response that has deepened alarm across Washington and exposed growing tensions within his own party.

In a series of posts on Truth Social, Mr. Trump condemned the lawmakers’ message as “dangerous to our country,” accusing them of engaging in “seditious behavior” and declaring that “their words cannot be allowed to stand.”

Moments later, he escalated further, writing that such actions were “punishable by death,” a remark that drew swift backlash from legal experts and elected officials alike.

The video at the center of the controversy featured six Democratic lawmakers, several with backgrounds in national security, addressing members of the armed forces directly. In measured language, they reminded service members that their oath is to the Constitution and that they are not obligated to carry out orders that violate the law.

“No one has to carry out orders that break the law,” one of the lawmakers said in the recording, echoing a principle long embedded in military doctrine.

To supporters of the lawmakers, the message was a reaffirmation of constitutional norms. But to Mr. Trump and his allies, it represented a dangerous intrusion into the chain of command — one that, in their view, risked undermining discipline within the armed forces.

The former president’s response, however, went well beyond criticism, alarming even some Republicans who have traditionally defended him.

“This kind of rhetoric is not helpful,” said one Republican strategist with close ties to congressional leadership, speaking on the condition of anonymity to discuss internal party dynamics. “It puts members in a difficult position and distracts from substantive policy debates.”

Indeed, the episode comes at a moment when signs of unease within the Republican Party are becoming harder to ignore.

In recent weeks, at least 18 Republican members of Congress have publicly diverged from Mr. Trump’s stance on key foreign policy issues, particularly regarding tensions with Iran. While disagreements within parties are not uncommon, the scale and visibility of the dissent have raised questions about the durability of the party’s alignment with the former president.

Several of those lawmakers have expressed concern, both publicly and privately, about the direction of U.S. foreign policy, as well as the tone of political discourse more broadly.

Behind closed doors, according to aides and advisers familiar with internal conversations, donors and former administration officials have also begun voicing apprehension. Some worry that increasingly combative rhetoric could alienate moderate voters and complicate efforts to maintain a unified front heading into upcoming elections.

At the White House, officials sought to contextualize Mr. Trump’s comments as a reaction to what they described as an unprecedented move by Democratic lawmakers.

“They are telling members of the military to ignore the commander in chief,” one administration official said, arguing that the video crossed a line. “That’s a serious matter.”

Legal scholars, however, offered a more nuanced view, noting that the obligation to refuse unlawful orders is well established under U.S. military law and international conventions.

“The principle itself is not controversial,” said a professor of constitutional law at Georgetown University. “What is unusual is the politicization of that principle in such a public and direct way.”

Democratic leaders, for their part, condemned Mr. Trump’s remarks as dangerous and irresponsible. Several called on Republican leaders to publicly reject the suggestion that elected officials should face extreme punishment for expressing legal interpretations.

“This is not how a democracy functions,” one senior Democrat said. “We debate, we disagree, but we do not threaten each other with imprisonment or worse.”

The clash highlights a broader transformation in American political life, where disputes that once unfolded through institutional channels are increasingly playing out in public, often in stark and personal terms.

For Mr. Trump, whose political style has long relied on direct communication and confrontation, the latest episode underscores both his enduring influence and the challenges that come with it.

While his base remains deeply loyal, the fractures emerging within Republican ranks suggest that maintaining cohesion may become more difficult as policy disagreements and strategic concerns continue to surface.

As Washington absorbs the fallout, one reality is becoming clearer: the pressures surrounding Mr. Trump are no longer confined to partisan opposition.

They are, increasingly, coming from within his own political coalition — a shift that could carry significant consequences in the months ahead.