City Council Speaker Corey Johnson has been sounding out Democratic insiders about running for mayor in 2021, sources said.

Johnson is on a long list of possible contenders hoping to succeed a term-limited Mayor de Blasio.

Although he has said he’s focused on his current position, Johnson is already laying the groundwork to be the progressive in the race by endorsing Democratic challengers to the state senators who were part of the Independent Democratic Conference, who are feeling the heat from the left for aligning with Republicans.

“Corey is clearly exploring it,” said one prominent Democrat.

Johnson, 36, is raising the issue by saying other people are encouraging him to run and asking Democratic insiders, “What do you think,” one source said.

“Corey would like to run … but he’s only been speaker for eight months. Let’s see how he performs over the next two years and see how his personality meshes with voters,” said another insider.

A Johnson spokesperson responded, “Corey loves being city council speaker and that is where his focus is right now.”