These rights are called "Weingarten Rights" based on a 1975 Supreme Court decision (NLRB vs. J. Weingarten). As with all rights, if we do not use them we lose them.
This statement could save your job:
"If this discussion could in any way lead to my being disciplined or terminated I respectfully request that my steward be present at the meeting. Without representation present, I choose not to respond to any questions or statements."