Liberal Democrat leader Tim Farron is to accuse Theresa May of pursuing the same "aggressive nationalistic" agenda as Donald Trump and Vladimir Putin.
In a keynote address to activists on Sunday, Mr Farron will say the Prime Minister is part of a "new world order" that links the US and Russian presidents and French National Front leader Marine Le Pen.
Speaking at the party's spring conference in York, he will say the Lib Dems are the "real opposition" to the Conservatives as the only party committed to fighting Mrs May's plans for a "hard Brexit".
In a deliberate echo of Tony Blair, Mr Farron will use his speech to try to reclaim patriotism for the centre left, telling his supporters they should not be "squeamish" about declaring their love for their country.
The Lib Dem leader will describe Mrs May as part of an "emerging consensus" on the political right characterised by nationalism, authoritarianism and protectionism, committed to reversing human rights and indifferent to or in denial of climate change.
"The politics of Trump. Of Putin. Of Le Pen. And now the politics of Her Majesty's Government. Welcome to the new world order. This is the new normal, the new status quo," he will say.
"Aggressive. Nationalistic. Anti-Nato. Anti-EU. It is the post-war internationalist consensus unravelling in real time. Winston Churchill's vision for a world that achieves peace through trade, common values and shared endeavour evaporating before our eyes."
He will urge his supporters to reclaim the British flag from nationalists who have turned it into a "symbol of division", saying they should never be ashamed of who they are.
"There are too many on the centre left who are squeamish about patriotism. I'm not. I love my country. I'm proud of my identity," he will say.
"Patriotism isn't about dividing our society. It is about celebrating it. It's about our shared experience, our shared history, our shared destiny.
"There's nothing logical about it. It's emotional. And it's good."
After a speech on Friday in which he accused Labour of acting as the Conservatives' "accomplices" on Brexit, he will say the Lib Dems are the only "real opposition" to the Tories.
"We are the only party in British politics opposed to a hard Brexit, fighting for our membership of the single market and campaigning to give the British people the final say over what comes next," he will say.