Why is Marco Rubio sliding in the polls?

The verdict was clear when Marco Rubio announced his presidential run with a well-received speech in Miami last April: There was a new member of the top tier in the Republican race. "The official 2016 GOP field gets its first real contender," wrote fivethirtyeight. "Marco Rubio is now a first-tier candidate," said Bloomberg's Mark Halperin. Washington Post columnist Michael Gerson placed Rubio, along with Jeb Bush and Scott Walker, in "the first tier of plausibility." It got even better from there. In June, the Washington Post declared, "Marco Rubio is now at the top of the Republican presidential field."


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