The Dropsite article says that 19 million people voted for Biden but not Harris. I'm not sure where they're getting that number from; Biden got
81 million votes in 2020 and Harris got
75 million in 2024, which is a difference of 6 million. To make up that difference she'd need 13 million votes from non-2020 voters or Trump 2020 voters which seems hard to believe.
Let's assume that the number of Biden/not-Harris voters is the difference between the two, 6 million. 29% of that is a little under 2 million votes. There were 154m votes in 2024 so it's a little over 1% of the total vote. If we use the higher number of 19 million then it's 3.5% of the total vote.
So if the poll is broadly accurate, then the people who said almost no one would care about Gaza were partly right: only a small group of people cared enough about the issue to sway their vote. But elections are won and lost by small margins, so small groups of people can have an impact.