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Rick Santorum threw his support behind presumptive GOP nominee Mitt Romney Monday, sending out an email to his supporters in which he endorsed the former Massachusetts governor. "Governor Romney will be that nominee and he has my endorsement and support to win this the most critical election of our lifetime," Santorum wrote. The endorsement comes after the two former rivals sat down for a one-on-one meeting in Pittsburgh last week where Santorum said he shared with Romney "the issues most important to conservatives." Santorum added, "I strongly encouraged Romney that he add more conservative leaders as an integral part of his team." Santorum dropped out of the Republican primary in early April, revealing that his campaign had run up against the money and power of Romney's well-oiled machine.