Agreed, although I suspect that the effect depends on which side of the cause you're on.

For those that agree with him, it will be more likely to reinforce their beliefs ("Yeah! Woohoo! Preach it, brother!"). See: dittoheads.

For those that are opposed, it more likely angers them and reinforces their opposition ("What a jerk! He's wrong AND a jerk!").

The neutral/undecided pack is where it gets interesting - they could be affected in any way: turning for, against, or remaining neutral.
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