Justin Baldoni's lawyer says Blake Lively's publicist is "dead in the water" because he has evidence of her alleged smear campaign against him ... but TMZ knows A LOT about how publicists work in Hollywood, and we made it clear ... the smear goes both ways.

The "It Ends With Us" director's attorney, Bryan Freedman, joined us on "TMZ Live" Wednesday and told us he's crafting up a lawsuit against Blake's PR person, Leslie Sloane.

Freedman says he has receipts showing Sloane planted negative stories about Baldoni in the press during Justin's beef with his "It Ends With Us" costar.

We challenged Freedman, explaining celebrity publicists always try to twist, steer and change the narrative in their client's favor ... and we would know, because we've been doing this for 19 years and have a deep understanding of how the game is played in Hollywood.

Freedman even acknowledges this ... telling Harvey he might end up being an expert witness if the lawsuit ever makes it to trial.

Check out the video ... We also asked Justin's lawyer what makes the alleged behavior here so egregious to warrant a lawsuit.

It'll be interesting to see what he puts in his lawsuit. Freedman insists he has lots of damning texts which he's going to release publicly on a website.

Stay tuned.