Suppose a person asserts that p. Suppose also that p entails q. Does it follow that the person asserting that p thereby asserts that q? If so, and if p and q are any propositions you like, then assertion is closed under entailment. If assertion is not closed under entailment, then there will be examples in which a person asserts that p, p entails q, but the person does not assert that q.
By 'entailment' I understand a relation between propositions. P entails q iff it is impossible for p to be true, and q false. By 'assertion' I mean a speech act, an act of asserting, a concrete, datable, linguistic performance, not a proposition. By 'the content of an assertion' I mean the proposition expressed when a person makes an assertion. A proposition is not the same as a sentence. 'The war has come to an end' is a sentence in English. 'Der Krieg hat zu Ende gekommen' is a sentence in German. The sentences are different, both at the type level and at the token level. And yet they can both be used to express one and the same thought. That same thought is the proposition. By 'thought' here I do not mean an occurrent episode of thinking, but the accusative (direct object) of such an act of thinking. You could also call it a 'content' although that term is ambiguous for reasons I won't go into now.
Preliminaries aside, back to our question.
That James no longer works for Amazon has among its entailments that James worked for Amazon, that someone named 'James' worked for Amazon, and that someone no longer works for Amazon.
Now suppose I assert that James no longer works for Amazon. Do I thereby assert that James worked for Amazon? I say No.
Here is a more striking example. Sophomore Sam asserts that there are no truths. The content of his act of assertion, namely, the proposition that there are no truths, entails that the content of his assertion is not true. But surely the latter is no part of what Sam asserts.
So assertion is not closed under entailment.
Suppose that Tom asserts that he is glad that Trump beat Hillary. The content of the assertion entails that Trump beat Hillary. But that Trump beat Hillary is not what Tom asserts. We can say that Tom's act of assertion presupposes that Trump beat Hillary. But neither Tom nor his act of assertion is a proposition. So if Tom's act of assertion presupposes that Trump beat Hillary, then presupposition is not a relation between propositions, but a relation between a non-proposition (a person or his speech act) and a proposition.
On the other hand, that Tom is glad that Trump beat Hillary entails that Trump beat Hillary. This is a relation between propositions and it makes some sense to say that the first presupposes the second.
This raises a question. Is presupposition primarily something that people do, or is it primarily a relation between propositions?