Rubio tweeted frequent updates on the shooting at the high school in his home state. Authorities say there are at least 17 killed.
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Rubio has received $90,205 in campaign donations from gun rights groups during the 2015-2016 campaign cycle and received an "A+" grade from the NRA. Rubio has received $3,303,355 over the course of his career as an elected official.
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"Today is that terrible day you pray never comes," Rubio tweeted