If this is true, then the effectiveness of the author's suggestion is significantly limited. Even if candidates start using the author's suggested technique, the media could just publicize excerpts from later on in a speech, where candidates are just arguing for their own positions, or even from the beginning, where candidates would seem to be arguing against their actual positions. So the benefit of the author's suggestion would be lost.

If anything, this strengthens the author’s position. If people find politicians’ arguments one-sided, then this technique would presumably mitigate that problem.

This might actually strengthen the argument. If using the author's suggested technique makes audiences view the speaker as fair-minded and trustworthy, that's a positive character judgment that might win votes.

This might also support the author’s argument, because using the technique the author recommends would then give audiences a more favorable view of political candidates.

We know from the stimulus that this technique is generally effective in making speakers more convincing to an audience. The stimulus doesn't tell us anything about whether this technique is more or less effective for different audiences or in different settings, so we can't make assumptions either way. This doesn't weaken the argument.