Three Democratic sources close to Rahm Emanuel tell CNN that the White House chief of staff informed senior colleagues he is all but certain to run for mayor of Chicago, and will leave the White House to take the final exploratory steps.

Close associates already are building a campaign team according to sources.

An announcement by Emanuel is expected to be scheduled for Friday, sources said.

One of the sources, a prominent Democrat close to the White House chief of staff, told CNN, “We see nothing that will stop a run. But you don’t announce a campaign for mayor of Chicago in Washington, D.C. You leave and go home and finish your business there.”

The second source said Democratic pollster Stanley Greenberg’s research of a possible mayoral run for Emanuel was “very positive. He has some obvious things to deal with, but it was very encouraging.”

Asked what the potential issues were, the source said “nothing you wouldn’t expect. Just reminding people Chicago always was before Washington.”

Assuming there is no hitch — and none is expected — longtime Obama adviser Pete Rouse is in line to be tapped as interim chief of staff, two of the sources said.

All of the sources spoke on condition of anonymity because of the sensitivity of the White House personnel turnover and the campaign planning.