We think that reading that a candidate's statement is false just as it is made could have quite a striking effect. It could trigger more visceral feelings among the viewers than standard fact-checking, which is published in separate articles. To over and over again read in the subtitles that what you're being told simply isn't true should outrage anyone who finds truth-telling an important quality.

Will the outrage be directed against the politician, or against the person who claims they're wrong?

I expect that any politician could take any speech made by a politician on the other side and "fact-check" it to produce a subtitled video "correcting" their "lies". How do you propose to establish a reputation for probity of the proposed system?