The longtime newsman hit the tech CEO with a lawsuit Thursday, in which he alleges EM refused to pay him after their deal fell apart. According to the docs, obtained by TMZ, Elon made a verbal promise to pay DL $1.5 million a year to make videos for X ... as a way to draw in dwindling advertisers.

Don claims he was also promised a share of the advertising revenue and other cash incentives if his presence on X (formerly Twitter) grew -- but Don's claiming Elon bailed on that agreement ... which Don believes violates the law.

While Don says in the docs that he didn't have a written contract in place, he claims Elon assured him over the phone about all these alleged promises -- and allegedly said he'd still support DL financially ... even if he didn't like the content.

Yet, as those familiar with the saga well know .... this deal soon crumbled after a disastrous interview between Don and Elon (Don's first official project with X) -- where the Tesla CEO responded poorly to questions about drug use and politics.

Elon later claimed on X he pulled the plug on the project because Don took a "dying" approach to journalism.

However, Don claims his future on X had already been threatened prior to the interview with Elon.

Don alleges he was pressured to announce his new collaboration with X at CES -- a famed tech conference in Las Vegas. DL claims his deal was to be revoked if he did not attend the conference alongside X CEO Linda Yaccarino.

Don's lawyer, Carney Shegerian, says this ... "X's executives used Don to prop up their advertising sales pitch, then canceled their partnership and dragged Don’s name through the mud." Don's suing for fraud, negligent misrepresentation, misappropriation on name and likeness, breach of contract and other causes of action. He's seeking damages.