These senators are Sanders' colleagues, and they know him better than the rest of us do. They may not have endorsed him, but they do caucus together. They are aware of the huge crack within the Republican Party thanks to the rise of Trump, as well as all the smaller cracks that have gotten even bigger ever since the party did not heed their own "autopsy" done after their second loss to Pres. Obama--and in fact doubled down on all their wrongdoing. Senate Democrats, as well as the rest of the Party, save one or two, knows that this crack is a huge opportunity, and one not to squander.
They are hoping to help ease Sanders away from his myopic crusade.
If Sanders continues to insist that he can still win even though Hillary Clinton is ahead of where she needs to be at this stage of the primary contest--he is doing so at the peril of the Party going into the GE. He is sticking with his smears and dog whistles--both helping the Republicans and keeping his supporters stoked and believing he can pull a giant upset by calling on the super-delegates to flip. Well, the super-delegates are not going to heed his call for them to switch their pledges, but Sanders continued smears against the front-runner in order to try to win are divisive and destructive. This might help Sanders continue with his campaign to win, but at the expense of damaging the Party and potentially throwing the election.
Sanders needs to responsibly lead his supporters--preparing them to vote in the GE for the nominee, which will be Sec. Clinton, and to help all the down-ticket Democrats running as well. This is the revolution we need.