On Friday morning, the group "Ready for Warren" released a letter calling on Sen. Elizabeth Warren to run for president — with the signatures of over 330 former Obama campaign staffers.

"We believed in an unlikely candidate who no one thought had a chance," the letter begins. "We organized like no campaign had organized before — and won the Democratic primary. We built a movement."

The letter calls "rising income inequality" the "challenge of our times", and concludes: "We want someone who will stand up for working families and take on the Wall Street banks and special interests that took down our economy … We urge Elizabeth Warren to run for president in 2016."

Warren's increased prominence

Warren has said repeatedly that she has no intention of running. But the new effort to coax her into the race comes as she's taken an increasingly prominent role in key political battles.

She has harshly criticized this week's government funding bill because it repeals part of Dodd-Frank, and is going to war with Obama over his nomination of investment banker Antonio Weiss for a position in the Treasury Department. She was also given a role in the Senate Democratic leadership in November.

The groups urging her to run

The progressive group MoveOn.org also called on its members to ask Warren to run for president this week. Another group, Democracy for America, is planning to join the "Draft Warren" movement as well — even though its founder Howard Dean announced this week that he'd support Hillary Clinton. (Dean remains an adviser to Democracy for America and his brother Jim chairs the group.) Several other high-profile former Obama staffers have endorsed Clinton too.

The groups are responding to genuine enthusiasm for Warren among their members. But Politico's Maggie Haberman pointed out that the groups' efforts to encourage Warren to run "serve an additional purpose — boosting their own fundraising and email lists."

Ready for Warren, the group that gathered signatures for the letter, is run by Erica Sagrans. She worked as a digital staffer in Obama's 2012 campaign, and has been working full-time on the effort to draft Warren. When I spoke to Sagrans last month, she said, "What happened in the midterms is very much a sign that we need Warren-type candidates. And that the thing you really need to start with as a candidate is really the authenticity and vision that Warren has that excites people. The money and everything like that can come later."