You may decide to hold off voicing your opinion if you want to gather your army first. People can contribute experience or information to your thinking — all the things that would make the disagreement stronger or more valid. 

Also, delay the conversation if you’re in a meeting or...

When your body language communicates reluctance or anxiety, it undercuts the message. 

Simply slowing the pace and talking in an even tone helps calm the other person down and does the same for you. It also makes you seem confident, even if you aren’t.

It’s a smart way to give the powerful person “psychological safety” and control. 

You can say: “ I have reasons to think that won’t work. I’d like to lay out my reasoning. Would that be OK?” This gives the person a choice, allowing them to verbally opt in.

Before you share your thoughts, think about what the powerful person cares about. You’re more likely to be heard if you can connect your disagreement to a “higher purpose.” 

State it overtly then, contextualizing your statements so that you’re seen not as a disagreeable underling but...

When you move on to expressing your concerns, watch your language carefully. Avoid any judgment words that might set off your counterpart. Share only facts.

Articulate the other person’s point of view. 

Stating it clearly, possibly even better than your counterpart did, lays a strong foundation for the discussion. You want your counterpart to say: "She/He understands."

Emphasize that you’re offering your opinion, not gospel truth. Remind the person that this is your point of view, and then invite critique. This will leave room for dialogue.