Good Amanda Becker piece for Reuters:

Legally, Super PACs are supposed to operate independently of candidates and cannot contribute to them directly. In reality, they often are run by donors close to the candidates and their campaigns.

David Donnelly, president of Every Voice, a campaign finance watchdog group, said candidates are smart to “think twice about winking and nodding at the Super PACs that are set up for themselves.” They “may be more of an albatross than a benefit” with Democratic primary voters, Donnelly said.

Promising to reject all Super PAC support, however, could cause consternation. Super PACs supporting Trump, the presumed Republican nominee, have continued raising money throughout his presidency.
Donnelly said that “you’d see a lot of hand-wringing in Washington about a candidate” who would forsake any Super PAC support, saying such a stance could be read as a “signal you’re not serious about winning.”