LOS ANGELES — 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 California before heading to Missouri on Wednesday, has not personally addressed the attack since London assigned blame to Russia. Aides released a statement in his name on Tuesday evening after he spoke with Prime Minister Theresa May of Britain by telephone expressing his solidarity.
“President Trump agreed with Prime Minister May that the government of the Russian Federation must provide unambiguous answers regarding how this chemical weapon, developed in Russia, came to be used in the United Kingdom,” the statement issued in Mr. Trump’s name said. “The two leaders agreed on the need for consequences for those who use these heinous weapons in flagrant violation of international norms.”
The British readout of the call attributed stronger language to Mr. Trump than his own White House statement did, reporting that during their conversation, “President Trump said the U.S. was with the U.K. all the way.”
The president made no further comment on Wednesday after Mrs. May expelled 23 Russian diplomats and vowed to crack down on Russian spies, corrupt elites and ill-gotten wealth in Britain. His first messages on his Twitter account on Wednesday concerned trade, infrastructure and his complaints that Senate Democrats are obstructing confirmation of his nominees.
A Trump administration official said Wednesday morning that the United States was working on a joint statement with Britain, France and Germany that should be issued later in the day and added that it would be strongly worded.
But Democrats and other critics of the president pressed him to speak out personally and possibly take action to back up Mrs. May.
“Where Prime Minister May has taken bold and decisive initial action to combat Russian aggression, our own president has waffled and demurred,” said Senator Chuck Schumer of New York, the Democratic minority leader. “Prime Minister May’s decision to expel the Russian diplomats is the level of response that many Americans have been craving from our own administration.”
Other critics noted that, under the NATO charter, an attack on one member is considered an attack on all.
“Judgment day for Donald Trump,” R. Nicholas Burns, a former ambassador to NATO and an under secretary of state under President George W. Bush, wrote on Twitter. Referring to President Vladimir V. Putin of Russia, he added: “Will he support Britain unequivocally on the nerve agent attack? Back #NATO sanctions? Finally criticize Putin? Act like a leader of the West?”
Evelyn Farkas, a former Pentagon official who oversaw Russia policy under President Barack Obama, said Mr. Trump should offer a range of assistance to Britain to help investigate the incident, prevent further such attacks on British sovereignty and impose punishment. She added that the United States could cite the suspicious death of Mikhail Y. Lesin, a former Russian minister, in a Washington hotel in 2015, in taking joint action. Investigators concluded that he died from a drunken fall but many remain skeptical.
“Frankly, I believe we should have and could still do this in response to Russia’s election interference in the United States and several other NATO countries,” she said. “We certainly should craft additional sanctions together with the U.K. and the E.U. to address the assassinations.”
Until Tuesday night, the White House had avoided pointing the finger at Russia for the attack, in which a former Russian spy was poisoned with a nerve agent near his home in southern England.
At her briefing on Monday, Sarah Huckabee Sanders, the White House press secretary, condemned the attack without publicly agreeing with Britain’s assessment that Russia was behind it. The administration’s only tough comment on Russian involvement until Tuesday came from Secretary of State Rex W. Tillerson, but he has since been fired.
By Tuesday morning, lower-level American officials joined in backing Britain as it retaliated against Russia.
“We stand in solidarity with our @NATO Ally, the United Kingdom, in condemning the offensive use of a nerve agent on their territory,” Kay Bailey Hutchison, the American ambassador to NATO, wrote on Twitter. “Russia must address UK questions & provide full disclosure of their chemical weapon program to @OPCW,” the Organization for the Prohibition of Chemical Weapons.
But the pattern resembles the way Mr. Trump has responded to the consensus finding of American intelligence agencies that Russia interfered in the 2016 elections. He has allowed top advisers to condemn Moscow for its election meddling but personally has used equivocal language in saying he accepts the conclusion — and generally expresses no outrage or criticism of Mr. Putin.
Asked about the meddling last week, after Robert S. Mueller III, the special counsel, indicted 13 Russians for spreading disinformation and propaganda in a concerted effort to influence the election, Mr. Trump focused on whether it changed the result, and avoided strong words about Moscow.
“Well, the Russians had no impact on our votes whatsoever,” he said during a news conference with Sweden’s prime minister. “But certainly there was meddling and probably there was meddling from other countries and maybe other individuals. And I think you have to be really watching very closely. You don’t want your system of votes to be compromised in any way. And we won’t allow that to happen.”
Even beyond Twitter, Mr. Trump will have opportunities to speak out on the attack in Britain if he chooses.
He plans to fly to St. Louis, where he will tour a Boeing plant and inspect warplanes it is making. He then will headline a fund-raiser for a Republican candidate running for Senate in Missouri before returning to Washington on Wednesday evening.