If there was clear evidence of a crime I assume they would "refer the matter to the Department of Justice" and expect DoJ to do their job.
The next step after that would be to demand that the president fire Sessions. Potentially impeach Sessions.
If the president refused to remove Sessions then the next step would be impeaching the president.
If Congress refused to act at that point, we have a pretty big problem.
In each case, obviously, the expectation would be that the clear and explicit defiance of the law would put more pressure on Republicans to act. That's why the "smoking gun" was important for Nixon.