LONDON — While Prime Minister Theresa May has united her warring Conservative Party by denouncing Russia over the poisoning of a former spy, things have not gone so smoothly for the opposition Labour Party. Among Labour lawmakers, familiar wounds have reopened over the more ambivalent stance of its left-wing leader, Jeremy Corbyn.

Twice in Parliament this week, Mr. Corbyn, whose political views were forged in his leftist activism in the 1980s, has eschewed the supportive, cheerleading role that opposition leaders traditionally play to the prime minister when Britain is in conflict with a foreign power. At times, in his reluctance to criticize Moscow, he sounded like his ideological opposite, President Trump.

While he condemned the attack in Salisbury, England, as an “appalling act of violence,” Mr. Corbyn has claimed that the Conservatives took donations from wealthy Russians, and he has highlighted cuts to the British Foreign Office and called for the Russian authorities to be “held to account on the basis of the evidence.”

On Thursday, in an article in The Guardian, Mr. Corbyn added that criticism of Russia should not mean acceptance of “a ‘new cold war’ of escalating arms spending, proxy conflicts across the globe and a McCarthyite intolerance of dissent.”