Update 5:00pm EDT:

In an email to supporters on Monday, Bernie Sanders highlighted four hotly-contested races that he says will likely determine the Senate majority: Pennsylvania, New Hampshire, Ohio and Nevada.

"The Koch brothers know this. That is why they are spending tens of millions of dollars to defeat these four candidates for Senate," Sanders wrote. "And that's why I'm asking you to support them: Katie McGinty in Pennsylvania, Maggie Hassan in New Hampshire, Ted Strickland in Ohio, and Catherine Cortez Masto in Nevada."

"I want to be clear," he continues. "It is very important that our movement holds public officials accountable. The Democratic Party passed an extremely progressive agenda at the convention. Our job is to make sure that platform is implemented. That will not happen without Democratic control of the Senate."

Earlier:

Bernie Sanders' highly-coveted email list of supporters—whose small donations smashed records and fueled his insurgent campaign for president—will not be handed over to the Democratic Party, campaign insiders say.

However, according to Roll Call, which cited an unnamed source who was "briefed on his plans," the progressive senator from Vermont will be utilizing that list and his wide popularity to raise funds and garner support for Senate hopefuls with an eye towards winning a Democratic majority come November.

Sanders "is preparing a post-Labor Day blitz of campaign activity," the news site reported Monday, which includes attending campaign rallies as well as soliciting funds. However, "unlike previous political forays, when the senator sent fundraising pitches on behalf of ideologically aligned candidates, this burst of activity is also expected to include more centrist Senate candidates locked in tight races against Republican opponents."

Similar to the reasoning Sanders gave when he endorsed Democratic rival Hillary Clinton, those close to the senator argue that "even incremental progress toward his progressive agenda would be blocked by a Republican-controlled Congress," as Roll Call put it, particularly in light of the Supreme Court vacancy.

However, the source added, "Anybody who has given to Bernie in 2016 can be rest assured that their info won’t be turned over to the [Democratic Senatorial Campaign Committee] or any other arm of the Democratic Party."

The news comes just days after the official launch of Our Revolution, which was inspired by the Sanders campaign and seeks to carry its progressive vision beyond the presidential election by providing support for local races and ballot issues.

However, because that organization was recently registered as a 501c4—a controversial move by Our Revolution president Jeff Weaver—Sanders and other federal officeholders are now restricted from coordinating with that effort, as Claire Sandberg, former organizing director for Our Revolution explained on Democracy Now! after its launch.