Tony Blair has called on Europe to recognise its "vital role" as a partner of the United States at a time when Western democratic values are under attack from outside and within.
The former prime minister said Europeans should not allow differences with "any one American policy or president" to undermine relations with the US.
Mr Blair, who is in Chicago to accept the Lincoln Leadership Prize, presented by the Abraham Lincoln Presidential Library Foundation, made clear that he still hoped Britain would remain part of the European Union, despite the vote to leave.
"Europe should understand its vital role in being America's partner irrespective of any differences with any one American policy or president; and the USA should see in the European Union - and I mean one united and - my hope - with Britain still in it - a vital supportive weight to help it counter those who challenge us," he said.
"In other words, now is the time to articulate our democratic values, champion them, celebrate them and fashion our politics so that we can preserve them."
Mr Blair's intervention comes amid concerns in Europe over President Donald Trump's threats to withdraw from the Iran nuclear deal and to impose tariffs on steel and aluminium imports.
He said it was important Western nations did not "turn in on ourselves" in the face of pressures from outside and within, and kept sight of the need to unite people around a workable vision of the future.
"Inside, our politics falls prey to false populism. Abroad, there is a new authoritarianism which looks at Western democracy and sees weakness and inadequacy. It offers a different, illiberal model of Government and argues that it can better deliver for the people," Mr Blair said.
"This model will be buttressed by growing economic and military power. It will exploit the insecurities of Western electorates worried whether the next generation will do worse than this, unconvinced of the virtues of immigration and tending to view globalisation as a project of the elite, not the people."
Mr Blair is the first British national to be awarded the Lincoln prize, which recognises individuals who have shown strength of character, individual conscience and a commitment to democracy.