The people most likely to watch a televised debate between political candidates are the most committed members of the electorate and thus the most likely to have already made up their minds about whom to support.
Conclusion (Hence)
Winning a televised debate does little to improve a candidate's chances of winning the election.
Intermediate Conclusion (Thus)
The people most likely to watch a debate have mostly already decided whom to support.
Evidence (Furthermore)
Debate watchers are the most committed members of the electorate.
After a debate, uncommitted viewers are generally undecided about who won.
Evaluate
How could we argue that winning a debate can do something significant to bolster your chances of winning the election?
The author's case covers two groups of viewers: 1. the ones most likely to watch are the most committed members of the electorate / they've made up their minds before they even watch the debate.
We could try to argue that even if you've "made up your mind", your voting decision could be changed by what you see in the debate, but that's a pretty hard angle.
2. The viewers who haven't committed to a candidate usually can't tell who won.
We could try to argue that even though they can't tell who won, they can tell who they liked more, and so that could lead them to vote for someone. But the conclusion isn't about appearing in a debate; it's specifically about winning a debate. So if they don't know who won, then it's hard for us to argue that the 'winner' gets a bump.
Are there any other voters to consider?
It seems like there are viewers who are somewhat committed (they lie in between "most committed" and "uncommitted"). Maybe they are swayed by the winner of the debate?
And then there are people who didn't watch the debate at all. Could they be swayed by hearing about who won the debate?
Goal
Since the question stem prefaces all the answers with "fails to consider", we can basically just treat this as a Weaken question.
Let's look an answer pointing to a way that winning a debate win could help a candidate.