EXCLUSIVE

The marriage of Khloe Kardashian and Lamar Odom is in crisis, and TMZ has learned ... the core reason is hardcore drug abuse.

Multiple sources familiar with the situation tell TMZ ... Lamar has had a problem for 2 years. It got so bad, last August Khloe pressured Lamar to go to rehab. Lamar went to a facility in San Diego, but he was so resistant Khloe hired private investigators to secretly stand guard at the rehab place to make sure he didn't leave. The P.I.s stood guard 24/7.

Our sources say Lamar constantly threatened to leave and Khloe made several visits to prevail upon him to stay. We're told after 3 weeks, Lamar left for good.

We're told Lamar actually stayed clean once he left rehab and was able to play well for the Clippers this past season. But when the season was over, sources say Lamar slipped deep back into hardcore recreational drugs.

As for reports that Lamar was cheating recently, we're told Khloe is convinced it happened, and she feels she's been hit with a double whammy ... but drugs are the main problem.

As for divorce ... neither Khloe nor Lamar has contacted a lawyer. As one source put it, "Khloe is more concerned about saving Lamar than ending her marriage."

We're told the situation has become dire. Sources tell us ... within the last 2 days, the entire family staged an intervention, trying to convince Lamar to go back to rehab, but he wouldn't have any of it.

In fact, on Wednesday morning, right after the failed intervention, Khloe threw Lamar out of the house and as you see from the pic (above), one of Lamar's friends is loading Lamar's stuff in his car as Lamar watches.