Nate Silver: Hillary does not have a problem on her left

[Bull Moose Base]

She has a problem with the left. Luckily for her the left is split to all hell (I'd like to see everyone get behind Sanders or Schweitzer myself), and the last progressive upstart is totally behind her.

Warren isn't totally behind Clinton. If she doesn't run herself, I doubt she'd endorse Hillary in a primary against Sanders. (Most likely she wouldn't endorse at all.) If Hillary's the nominee, obviously Warren will endorse or campaign for her.

[Bull Moose Base]

Warren accepted a request to sign an ostensibly private letter to Hillary encouraging her to run. That's not an endorsement really. Warren probably guessed it was likely to leak to the public but if anything that diminishes it since it's a good bet she just didn't want to invite a media frenzy. (Same with Klobuchar) When asked about Hillary, Warren is generally positive (as is Bernie Sanders) but largely dodges. I'd find Warren endorsing Hillary in a primary more surprising than Warren running herself.