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Pete Buttigieg is urging Bernie Sanders supporters to get on board with Joe Biden, and quickly, because -- clearly referencing the coronavirus response -- he says it's a matter of life or death to defeat President Trump.

The former Democratic presidential candidate joined us Friday on "TMZ Live" and pushed for unity within the Democratic party as Biden and Sanders prep for Sunday's debate. Up to this point, many Bernie backers ain't warming up to Biden, but Pete says now's the time for that to change.

He says the debate will be tricky waters for both candidates to navigate, because if they both go for the jugular ... Trump will be the only winner.

The way Pete sees it, the differences between Biden and Bernie are molehills compared to the mountainous divide across the aisle -- and he says Trump is failing to keep American lives safe in the face of the deadly pandemic.