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

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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.

The Romney campaign, which has long traveled with private security of its own, did not request Service protection, and was approached about the additional security steps in the weeks following the New Hampshire primary, the sources told ABC News.

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