Question

What important information is omitted from this ad?

"The senator received fifteen letters against the bill."

whether fifteen letters is a large number received for one bill
how long the letters were
how well-written the letters were

Answer 1

Main omission: whether fifteen letters is a large number (or otherwise significant) for one bill.

Why: the bare count means little without context — typical volume of mail on that issue, the time period, whether the letters were unique or form letters, whether they came from constituents, and whether there were comparable pro letters. Those facts determine whether 15 indicates meaningful opposition.

The other two items (how long the letters were; how well‑written they were) are irrelevant to the claim that the senator "received fifteen letters against the bill."