Here's the deal ... the union representing USCP officers says many officers are leaving the force soon, and at least one of those officers tells us why the departures could spell trouble for Capitol Hill.

The USCP officer, who is planning to leave the department by the end of the year, tells TMZ ... the sheer number of officers planning to leave the department will put the Capitol at more risk and could make the building more vulnerable to another riot or attack in the future.

We're told there are around 20 USCP officers leaving in the next 6 months, and they're packing their bags because of how the department has been pointing fingers since the riots, with little to no accountability from the top down.

The way this outgoing officer sees it, there are 2 factors that could lead to another January 6th ... less manpower, and a high degree of uncertainty from officers on what to do the next time a mob were to attack.

Part of that uncertainty, we're told, involves the use of deadly force. We're told USCP officers are still scared to use their guns, for fear of being suspended like the officer who shot and killed Ashli Babbit.

As we first told you ... a deadly force policy still remains unclear within USCP, and we're told there has still been no communication between top brass and rank and file as to how to handle another insurrection.

Making matters worse, we're told, is the union's overwhelming 92% vote of no confidence for acting chief Yogananda Pittman, who got the job 2 days after the insurrection.

It sounds like USCP can't do much to stop the impending exodus among the rank and file ... we're told the officers are just tired of working there with all the "organizational dysfunction."