The Secret Service is moving forward on activating 24-hour protection for Republican presidential candidates Donald Trump and Ben Carson, Fox News has learned. 

The agency sent letters Monday to key leaders in Congress who have to sign off on the proposal, which Fox News is told would involve a rotation of 260 Secret Service agents. Congress traditionally approves such requests. 

The development comes after an aide to Carson met with the U.S. Secret Service's Dignitary Protection Division to request protection for the candidate as threats against him have grown, according to a source familiar with the conversation. 

The source told Fox News that a representative for Trump's campaign also made contact with the Secret Service on Friday. 

Trump last week was outspoken in calling for Secret Service protection, saying that while he has a private security team, his rallies have gotten large. 

"We're getting the biggest crowds," he told Fox News' "On the Record with Greta Van Susteren" last Thursday, acknowledging his team has been speaking with the Secret Service while suggesting a double-standard was at play for Republican and Democratic candidates. "If I was a Democrat, they'd have it but it's really up to them. They know what they're doing, and I assume they're doing the right thing. I would hope they are doing the right thing."

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Newsmax first reported over the weekend that the two GOP candidates were likely to get Secret Service protection amid heightened threats against them. 

While it is rare for such official protection to be given to candidates before they officially get their party's nomination, the Secret Service will make exceptions based on threats. For example, then-Sen. Barack Obama received protection early in 2007, long before he secured the nomination, amid an uptick in threats against the man who would be the first black president. 

Former Secretary of State Hillary Clinton has had Secret Service protection dating back to her days as first lady, and has that protection on the campaign trail now.

Ed Henry currently serves as Fox News Channel's (FNC) chief White House correspondent. He joined the network in June 2011.