TL;DR: You.
Longer Answer:
All the time, but more frequently and urgently as each presidential election draws near, analysts and pundits agonize over which “major party” candidate is hurt most by independent and third party candidates: Will, for example, the Green nominee “spoil” the election for the Democrat and “hand it to” the Republican, or will the Libertarian nominee have the opposite effect?
The controversy of the moment concerns a group called “No Labels,” which may attempt to put a presidential candidate on government-printed ballots in 2024. “Democratic strategists and anti-Trump Republican operatives have concluded,” David Corn writes at Mother Jones, “that its effort could siphon more votes from President Joe Biden than Donald Trump.”
But the only votes in Biden’s and Trump’s gas tanks are their own (and the No Labels candidate is unlikely to receive either of those votes).
Your vote is in your gas tank, and belongs to you — not to Joe Biden, Donald Trump, or anyone else — until you cast it.
It’s arrogant of those analysts and pundits to assume that Democrats and/or Republicans own or are owed your vote as a matter of entitlement because, well, they’re the big guys, and that you’re essentially stealing it from them if you cast it for some other candidate. They’re not entitled to your vote, and when you cast that vote for the candidate you evaluate as having earned it, it doesn’t come “from” some other candidate.
Naturally, I hope to be your candidate of choice. But if I’m not, I won’t complain that Joe Biden, Donald Trump, Cornel West, or some other candidate “siphoned” your vote “from” me.