I think I can see what you're trying to say, but you will need to change it more than you have: If you can't even tolerate opposition on the Internet, then you won't be able to tolerate democracy, either. "Endure" is also possible, but "tolerate" is the meaning you want, I think. "Sustain" is wrong for the first clause.
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MIGwhy you changed opponent to opposition?Because what you're tolerating is the actions of the opponent, opposition, not the opponent himself.
MIGI think. "Sustain" is wrong for the first clause.because of its meaning? because it can also express "supporting"?For the meaning, and yes, because it is certain to be misunderst