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But Kaine predicted Sanders will eventually embrace Clinton.

Sanders has ramped up an aggressive campaign ahead of the California primary on June 7, in which 475 pledged delegates are at stake.

He has revved up supporters by called the Democratic primary process “a rigged system” warning that it would be a “disaster” if Clinton won the primary and picked a moderate or conservative running mate.

Sanders needs to win the California primary by a significant margin to give him momentum heading into this summer’s Democratic convention.

If Clinton’s numbers continue to fall, Sanders hopes he can persuade delegates at the convention to switch to him as the more viable general-election matchup against Trump. Clinton suffered a setback last week when a State Department audit found that she inappropriately handled government emails on a private server while serving as secretary of State.

Top Democrats are worried that even if Sanders backs Clinton in the end, his thousands of young progressive supporters may not be enthusiastic about her as the nominee, depressing voter turnout.

Democratic leaders are hoping that their party will unify in opposition to Trump in November, but Kaine warned that fellow Democrats shouldn’t underestimate the billionaire businessman, who has consistently defied pundits’ predictions.

He told CNN that his party should have “zero complacency.”

“With Trump I’ve always thought it would be unpredictable. You have to assume going in there could be places where we assume victory that suddenly we have to work harder for, but there are also places where the Republicans have assumed victory in the past that suddenly come onto the map and come into play because of Trump,” Kaine said.

“So his candidacy puts a lot of crosswinds into an electoral map,” he added.