Federal bills passed by Congress do not need the President’s signature to become law. Any unsigned-but-unvetoed bill automatically becomes a law ten days after being sent to the President unless Congress is out of session, which it almost never officially is nowadays.
Since a Trump win would probably result in a GOP Congress that could decide to exempt abortion restrictions from filibusters, his current “I would not sign a federal abortion ban” statement may not even be a lie — it doesn’t need to be.
So he should be pushed to clarify his veto stance, which he may be uncomfortable doing since that might dampen evangelical support.