Trump is an existential threat to Iran. Iran is in a regional cold[0] war with Israel. Israel’s ability to wage this war is largely dependent on US support; both in terms of raw military assistance, and in the US providing diplomatic and economic cover for Israel.
While the US had not applied nearly as much moderating pressure on Israel as I would have liked, it has still provided some. Israeli prime minister Netenyahu, in contrast, has been angling for a direct confrontation with Iran for decades now. Given the past 11 months, there are serious forces, both in Israeli politics, Iranian politics, and the inertia of war, pushing in that direction.
Trump is aligned with Netenyahu on this point, and would push him towards a direct confrontation with Iran. By all indications, Harris is not. This dynamic was made clear to Iran when the Democratic administration signed the Iran nuclear deal (against Israeli opposition), from which the US under Trump proceeded to unilaterally withdraw from.
[0] Cold might be a bit of an understatement after the last 11 months. However, apart from a brief tit-for-tat exchange, the fighting has stayed confined to Israel and Iranian proxies.
Part of plausible deniability is that it has to be plausible. There has been no plausible argument presented that Israel did not do the pager and walkie talkie attack. For that matter, there hasn’t even been a denial about it.