While I'm only adding to the choir of people telling you "of course," since I'm directly the person you're responding to it still feels worth saying: yes, of course, if America and Venezuela went to war, it's completely legal for Venezuela to attempt to kill the U.S. President.

As an American, I certainly hope they would fail. But do I think it's legal? Yes: it's a targeted strike on the leader of an enemy country they'd theoretically be at war with. Do I think it's wise? Well — no, Venezuela has a much smaller military, and assassinating the U.S. President would trigger a massive war that would devastate Venezuela for decades while modestly inconveniencing American taxpayers. But legal? Yes.

They could do that now and it might be legal under international laws of war.

We've massed forces for an attack, attacked their ships, violated their airspace with combat aircraft (that's today), and extensively and publicly threatened them. They'd be in their legal rights to strike preemptively, including possibly a decapitation strike (this is why the Dubya administration kept repeating the term "preemptive strike", even though it was obviously nowhere near applying in the case of Iraq—it was a way of asserting its legal basis)

[edit] As thereisnospork points out in a sibling comment, however, this doesn't mean it'd be a good idea.

If US and Venezuela are in a state of war, then the head of the US Armed Forces is a legitimate target.

Not sure why you have doubts about this.

The US and Israel do the equivalent of that and have been for years. An assassination is an assassination. The weapon makes little difference.

I mean of course they could, and should[0] how is that a question?

[0] Shouldn't - classic example of a tactical win being a strategic blunder. Killing the American president and would solidify American public support for the war - which would probably be undesirable in the balance.