Washington (CNN) Bernie Sanders' campaign invoked Robert F. Kennedy in a Twitter comeback to Hillary Clinton on Tuesday.

For four minutes.

Then, the tweet was deleted. And the campaign hasn't responded to questions about why.

Clinton's campaign was live-tweeting the former secretary of state's speech on racism, piece-by-piece.

One of the @HillaryClinton account's tweets said: "Anyone asking for your vote has a responsibility to grapple with reality—to see things as they actually are, not just as we want them to be."

Anyone asking for your vote has a responsibility to grapple with reality—to see things as they actually are, not just as we want them to be. — Hillary Clinton (@HillaryClinton) February 16, 2016

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