Former National Security Agency contractor Edward Snowden has ridiculed presumptive Republican presidential nominee Donald Trump for backing out of a debate with Democratic rival Bernie Sanders.

Trump and Sanders have previously expressed an interest to hold a one-on-one encounter in California, in what could have been an unprecedented move since Republican and Democratic candidates traditionally do not debate each other until the parties have selected their nominees.

However, Trump rejected the idea in a surprise move on Friday, attributing his decision to Democratic frontrunner Hillary Clinton's lead in her party's primary race.

"Based on the fact that the Democratic nominating process is totally rigged and Crooked Hillary Clinton and (Democratic National Committee chair) Deborah Wasserman Schultz will not allow Bernie Sanders to win, and now that I am the presumptive Republican nominee, it seems inappropriate that I would debate the second place finisher," he said in a statement.

Snowden, the NSA whistleblower, echoed Sanders Saturday, tweeting that, “Who would have imagined Donald Trump’s greatest fear was an economic debate?”

After Trump said he would not hold such a debate, Sanders asked the business mogul, “What are you afraid of?”

The debate would have become a potentially high-ratings television spectacle and it could have been a big boost to Sanders' chances in the California state primary on June 7.

Sanders is running behind Clinton in the race for the Democratic nomination for the November 8 presidential election.