ST. LOUIS — Britain’s tough response in holding Russia responsible for a poisoning attack on its soil increased the pressure on President Trump to join with a NATO ally in taking action, even as he has been reluctant to retaliate for Moscow’s intervention in the 2016 election in the United States.

Mr. Trump, who was visiting Missouri on Wednesday, has not personally addressed the attack since London assigned blame to Russia and left it instead to aides to express public solidarity with Prime Minister Theresa May after she expelled 23 Russian diplomats, canceled high-level contacts and vowed to impose more sanctions.

“This latest action by Russia fits into a pattern of behavior in which Russia disregards the international rules-based order, undermines the sovereignty and security of countries worldwide, and attempts to subvert and discredit Western democratic institutions and processes,” the White House said in a written statement. “The United States is working together with our allies and partners to ensure that this kind of abhorrent attack does not happen again.”

But for whatever reason, Mr. Trump avoided saying so personally in public, much as he has generally avoided condemning Russia for its election meddling. He has allowed top advisers to denounce Moscow for its interference in American democracy, but when it comes to his own Twitter posts or comments, he has largely stuck to equivocal language, seemingly reluctant to accept the consensus conclusion of his intelligence agencies and intent on voicing no outrage or criticism of President Vladimir V. Putin of Russia, for whom he has expressed admiration.