BRUSSELS (AP) — Britain's referendum campaign grew increasingly edgy Tuesday after Prime Minister David Cameron suggested that a British EU exit would make the Islamic State group happy and the EU president lashed out at London's ex-Mayor Boris Johnson for comparing the bloc's aims to those of Adolf Hitler.
The arguments sparked allegations of rhetorical excess from both sides and prompted European Council President Donald Tusk to finally enter the referendum debate, deriding Johnson's comments.
Tusk said that "when I hear the EU being compared to the plans and projects of Adolf Hitler I cannot remain silent."
"Such absurd arguments should be completely ignored if they hadn't been formulated by one of the most influential politicians of the ruling party," Tusk said in Copenhagen.
Johnson argued over the weekend that the 28-nation bloc was creating a super-state that mirrors the attempt of the Nazi leader to dominate the European continent.
Britain holds a "leave or stay in" referendum on June 23 on its always cantankerous relationship with the EU, and the campaign has become increasingly edgy.
In a speech Tuesday aimed at bolstering arguments for remaining in the EU, Cameron said "it is worth asking the question: Who would be happy if we left?"
He said Russian President Vladimir Putin "might be happy" and that he suspected IS leader Abu Bakr "al-Baghdadi might be happy."
Cameron said that friendly countries around the world "are saying 'we would like you to stay, we think it's good for us and it's good for you.'"
Johnson countered that "it's a bit much ... to say our allies are Putin and Daesh," another name for the Islamic State group.
In Britain, the EU institutions and its leaders in Brussels have long been seen as the source of all things bad related to the bloc. Because of it, Tusk and others have taken a hands-off approach in a campaign that is also vital to the future of the EU itself.
That changed for Tusk when Johnson, the most prominent British politician on the "leave" side, said the past 2,000 years of European history have been dominated by doomed attempts to unify the continent, including those of Napoleon and Hitler.
"Boris Johnson crossed the boundaries of a rational discourse, demonstrating political amnesia," said Tusk.
Britain's "stay" camp described Johnson's comments as a desperate effort take the focus off the economic impact that such a rupture would create and shift it to the more populist theme of sovereignty issues.
By evoking World War II, Johnson reminded much of the country of its "finest hour" — the moment that Britain acted as the bulwark to halt Nazi tyranny — and he stressed the value of sovereignty over being part of a multinational effort to run Europe's affairs.
Tusk said, however, that "the EU still remains the most effective firewall against the ever-dangerous and often tragic conflicts among the nations of Europe."
"The only alternative for the Union is political chaos, the return to national egoisms," Tusk said.
Jill Lawless reported from London.