Bernie Sanders, Senator for the state of Vermont and former candidate to run against Mr Trump this year, is due to make the comments in a speech this week at the Democratic National Convention. The democratic socialist is expected to claim another Trump presidency will threaten democracy.
In a speech released early by Convention organisers, Mr Sanders will say: “This election is the most important in the modern history of this country.
“In response to the unprecedented set of crises we face, we need an unprecedented response – a movement, like never before, of people who are prepared to stand up and fight for democracy and decency—and against greed, oligarchy and authoritarianism.
“The future of our democracy is at stake. The future of our economy is at stake. The future of our planet is at stake.
“We must come together, defeat Donald Trump and elect Joe Biden and Kamala Harris as our next president and vice president. My friends, the price of failure is just too great to imagine.”
Mr Sanders has run twice as a presidential hopeful – once in 2016, when Hillary Clinton ended up securing the Democrat nomination – and again this year.
Mr Sanders’ run was described by analysts as youth-run and focussed on working-class social change, though he eventually fell behind Mr Biden in polls before dropping out of the race.
Mr Trump later called on Mr Sanders’ supporters to “come to the Republican party” despite political differences.
READ: US election shock: Donald Trump outlines key reason why he should ‘redo’ last four years
It comes as a recent poll conducted by research company SSRS revealed Mr Trump may be narrowing the gap between himself and Mr Biden.
For much of the year, polls have shown Mr Biden holding a steady lead over the current president in national opinion.
This newest poll showed 50 percent of respondents would vote for Mr Biden in the election, compared to 46 percent for Mr Trump. Other did not answer or chose another option.
It marks a swing in favour for Mr Trump, as the same poll conducted in early June showed 55 percent would vote for Mr Biden and 41 percent would vote for Mr Trump.
The new SSRS poll involved 1,108 respondents and was conducted between August 12 and August 15.
Mr Biden still appears to hold a lead over Mr Trump in the majority of polls, however.
Mr Trump is currently on a run of campaign rallies in which he will discuss topics including jobs and the economy.
He held one yesterday in the state of Minnesota – only two days after Mr Trump announced the death of his younger brother Robert Trump.