To support his line of argument, he pointed to three policy positions Clinton had taken in the past: supporting the invasion of Iraq, remaining silent for years on the Keystone XL pipeline and backing the initial negotiations of the Trans-Pacific Partnership agreement.

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In the case of TransCanada's permit application to build a massive oil pipeline between Canada and the United States, he wrote, "Most progressives that I know were opposed to the Keystone pipeline from day one. Honestly, it wasn’t that complicated." Tommy Vietor, who served as Obama's National Security Council spokesman before leaving to start the San Francisco-based consulting firm Fenway Strategies, shot back:

Sanders tweeted "Most progressives I know are firm from day 1 in opposition to the Trans-Pacific Partnership. They didn't have to think about it a whole lot." Vietor countered, "Obama is a progressive who supports TPP."

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Vietor, who has 12,800 Twitter followers, also questioned the senator's overall premise, suggesting that most Americans reject strict ideological litmus tests:

Ben LaBolt, who served as the national press secretary for Obama's 2012 reelection campaign and now manages the New York-based consulting shop he co-founded, The Incite Agency, weighed in on Vietor's side in the dispute.

Obama has officially not taken sides between the two Democratic primary candidates, although he is a strong proponent of the Pacific Rim trade accord and took years to veto Keystone XL. And the president took pains in a recent Politico interview to praise Clinton's approach to governing and rejected any comparison between himself in 2008 and Sanders during this current campaign.