Former Vice President Joe Biden is now being protected by the U.S. Secret Service, a law enforcement source told Fox News on Tuesday.

Biden, the front-runner for the 2020 Democratic presidential nomination, had the protection activated this week after earlier requesting it. The development comes not only as Biden moves closer to the nomination but as protesters have disrupted several recent events -- even rushing the stage and prompting staff and family members to intervene to help protect the candidate.

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The law enforcement source described a multi-stage approval process, telling Fox News that a candidate first needs to qualify for protection, then request it from a select congressional committee and the Department of Homeland Security. Once approved, the U.S. Secret Service is authorized to protect the individual.

Biden's rival for the Democratic nomination, Sen. Bernie Sanders, I-Vt., has not yet had Secret Service protection activated, the source told Fox News.

It is rare for official protection to be given to candidates before they officially get their party’s nomination, but the Secret Service makes exceptions based on threats.

For example, in 2015, the Secret Service activated 24-hour protection for then-candidate Donald Trump and then-candidate Ben Carson after both received threats; and then-Sen. Barack Obama received protection in 2007, long before he secured the nomination, amid an uptick in threats against him.

But former Secretary of State Hillary Clinton had Secret Service protection dating back to her days as first lady, and had that protection on the presidential campaign trail in 2016.

Biden, as vice president, also had Secret Service protection.

While it is unclear, at this point, if the Biden campaign has received any sort of threat, earlier this month, the former vice president had protesters storm the stage during a victory speech.

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Two female protesters rushed the stage, shouting: "Let dairy die." They held posters reading the same and showing the image of a cow.

Dr. Jill Biden, and Biden’s sister, Valerie, attempted to block the protesters from touching the former vice president.

Campaign senior adviser Symone Sanders immediately hit the stage, grabbing a protester. The other protester was dragged off the stage by security. Biden continued on with his speech.

Sanders tweeted, later, saying: "I broke a nail. #SuperTuesday"