The Hill reports that "President Obama on Thursday warned Democrats against adopting a “Tea Party mentality” that could lead to deep divisions within the party and harm its chances of winning national elections." Link
This is an obvious reference to Bernie Sanders and his supporters, for several reasons.
1) Timing: Sanders just created a controversy by calling Hillary Clinton unqualified to be president. Since the President obviously believes her to be qualified, Obama had a big problem with Sanders' comment.
2) Obama referenced those who have been pushing for single payer. I.e. Bernie et. Al.
3) Obama scolded those who call other democrats corrupt. Hillary hasn't gone there. Bernie has.
“And that anybody who suggests there is another point of view ... well, then you must be a sellout or you must be corrupted or you must be on the take or what have you," he added. "That is not, I think, useful.”
Heed The President's advice.