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TAMPA - Mitt Romney's presidential campaign has been informed that it will start receiving Secret Service protection this week, two campaign sources and a senior Republican tell ABC News.

Secret Service protection is being given to the campaign not because of a specific threat but because of the increase in crowd sizes as the primary season has progressed over the past few weeks, according to the sources, who refused to be identified because they don't have authority to comment on such matters publicly.

According to a federal law enforcement officials Romney's campaign, which has long traveled with private security of its own, did request Secret Service protection. However sources tell ABC that the campaign did not make the request, but was approached about the additional security steps in the weeks following the New Hampshire primary.

The Secret Service is charged with protecting presidential candidates in addition to the president himself, and providing protection to a candidate before he secures a nomination has become fairly common. Such decisions are made by the Department of Homeland Security, in consultation with congressional leadership.

Officials with the campaign and the Secret Service declined to comment on the decision.

Romney will become the only current Republican candidate with Secret Service protection. Herman Cain became the first candidate to receive Service protection, in November, but he ended his candidacy in December.