Donald Trump -- Smoking Gun Video Shows Reporter Cross the Line (VIDEO)

TMZ has obtained the video that became a get out of jail free card for Donald Trump's campaign manager.

The surveillance video shows what happened before Corey Lewandowski grabbed reporter Michelle Fields. You see a secret service agent telling her to move away from a secured area and go back to the media area.

Fields walks away, stands to the side for about 1 minute and 40 seconds, and then makes her move. It's at about 2:30 in the video ... Fields walks away as if she's leaving the room, and then comes back -- when the agent turns his head -- and beelines it back to the secured area.

As we reported, an expert witness filed a declaration with prosecutors saying what Lewandowski did was not out of the ordinary. As a result, the case was dropped.