• Massachusetts senator Elizabeth Warren has formed an exploratory committee to run for president and is visiting the early voting states of Iowa and New Hampshire.
• California senator Kamala Harris has closed her state campaign committee and is on a publicity tour for her new book.
• Senators Sherrod Brown of Ohio and Kirsten Gillibrand of New York are planning trips to Iowa.
• Many in the media continue to advise Joe Biden against running, most recently The Boston Globe, The Economist, Vox and WGBH.
• Independent Vermont senator Bernie Sanders is shaking up his staff to make it less male and white.

Winnowing

Five prominent Democrats have announced they are not running:

• Michael Avenatti, the celebrity lawyer
• Andrew Cuomo, the governor of New York
• Martin O’Malley, the former governor of Maryland
• Deval Patrick, the former governor of Massachusetts
• Tom Steyer, a billionaire hedge-fund manager

Bloomberg View columnist Jonathan Bernstein counts 31 Democrats who have run so far, with 25 still in the mix.

He expects the field to winnow down to around a dozen by the time the first televised debates are held in June.

What is the invisible primary again?

Bernstein has a good summary in his latest column:

For the past two years, as many as three dozen Democrats have been trying to line up support among party actors — politicians, campaigning and governing professionals, formal party staff and officials, activists and donors, party-aligned interest groups and the partisan press — and now we’re finding out the first results of that competition. By the time voters have a say in February 2020 (or earlier; the calendar isn’t final yet), a good deal of sorting out will have been done. And those party actors will have given some candidates considerable resources and others very little.

Read more

• Josh Putnam on the significance of California moving up its primary by three months.
• Seth Masket on how the “party decides” theory is holding up.
• Nate Silver on the five constituencies in the Democratic Party.
• Geoffrey Skelley on why primary debates, unlike general-election debates, matter.