Elizabeth Warren and Donald Trump have been engaged in a verbal battle for months - but it seems they might agree on one thing.

Speaking at a rally in Denver on Sunday, the Clinton campaign surrogate said: "Donald Trump has been talking a big game, about how the game is rigged. Well, I want to say one thing.

"He’s right. It is rigged. It is rigged for the Donald Trumps and the billionaires of the world and we are here to fight back."

Mr Trump’s national numbers are trailing as much as 11 points behind Hillary Clinton, according to the latest NBC News / Wall Street Journal poll. He has since accused the government of widespread voter fraud, prompting his critics to suggest he is already aware he might lose the election. He also said the fact that his rival was not in jail was evidence the election was fixed.

"We’re fighting back against a basic GOP philosophy to help the rich and powerful get more rich and powerful and step on everyone else along the way," Ms Warren said.

Ms Warren imitated a chicken’s clucking, adding that the nominee was too “chicken" to release his tax returns.

She also called him a "sleaze" and a "cheapskate", continuing a narrative she has pursued for months.

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Ms Warren and Bernie Sanders have joined forces to woo hesitant and disillusioned millennial voters, just three weeks before the election.

Ms Warren praised Bernie Sanders for bringing about the Democrats’ "most progressive policies in history".

"To every person who felt the burn during the primary, America and the Democratic party know a power and energy of the progressive movement," she told the crowd.

She added that Mr Sanders had made Hillary Clinton a “stronger nominee” and would make America “stronger country”.

Ms Warren said she was “proud” to be a Democrat.

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"I am proud that we are a party that doesn’t debate who has the smallest hands. Or who can build the longest, tallest, fake gold-plated wall in the world," Ms Warren said.

"I’m proud that we are not looking for the most obscene way to demean women. I’m proud that Democrats spend our time working on how we can build a future for all of our people."

When Mr Sanders took the stage, he would know that younger voters are flocking to independent candidates such as Jill Stein and Gary Johnson instead of Ms Clinton.

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"I have been running all over this country for a year and a half talking about a corrupt tax system. And in one day, Donald Trump did more to explain the corrupt tax system to the American people than I did in a year and a half. Thank you, Donald," he said.

"What Donald said is, ‘Yeah, I’m a multibillionaire. I’ve got mansions all over the world. But you know what? I, a billionaire, don’t have to pay any taxes and I’m proud of it because you suckers are going to pay the taxes for me,'" Mr Sanders added.

Mr Trump’s 1995 tax return showed that he made a loss of almost $1 billion, which could signal that he carried a loss over twenty years to avoid paying federal income tax.

"We’re going to beat Trump and beat him badly," Mr Sanders said.