When the Trump administration proposed a peace plan that would recognize Russian rule of the Crimean peninsula, the response from Kyiv was a loud and unequivocal no.

Doing so would violate the nation’s Constitution, President Volodymyr Zelensky of Ukraine told reporters. It would never happen, he declared, not even in exchange for the end of the bloody war raging mostly away from the disputed territory that has been in Russian hands for more than a decade.

Mr. Zelensky’s red line has a hard political reality holding it in place.

Inside Ukraine, formal recognition of Russian control of Crimea would be widely viewed as a dangerous concession to a duplicitous rival and an abandonment of Ukrainians still living in the region. It would also dash hopes for reunification of the families separated by the 2014 occupation — when many pro-Ukrainian residents fled while their elderly or pro-Russian relatives remained behind.

“There is not a single Ukrainian politician who would vote to legalize the occupation of Ukrainian territories,” said Kostyantyn Yeliseyev, former presidential deputy chief of staff. “For members of Parliament, it would be worse than political suicide,” he said.

President Trump expressed bewilderment and frustration at Mr. Zelensky’s reaction on Wednesday, posting on social media that Crimea was “lost years ago” and suggesting that the Ukrainian leader was prolonging the war over a pipe dream.

“He can have Peace or, he can fight for another three years before losing the whole Country,” Mr. Trump wrote.