I 100% agree it’s a losing topic for them. Which is why I have some concern that placing that issue on the ballot separately may benefit them.
I am aware that ballot measures regarding abortion have yielded overwhelmingly favorable results with regard to protecting abortion rights. But I am not sure that all those people who have turned out to vote in favor of abortion rights/against abortion bans in places like Kansas, for example, wouldn’t otherwise vote Republican. Are there results showing that where abortion rights have been placed on the ballot, there was a notable shift in votes favoring democrats compared to prior elections?
By the way, what I am discussing here is not my hypothesis; just a concern.
538 did a piece on this topic a few weeks ago. Their conclusion was that there is very little evidence that ballot measures have much impact on the other races at all.
It could be this factor (i.e. pro-choice voters feeling less guilt) cancelling out the increase in turnout from the pro-choice voters. Or it could be that neither factor makes much of a difference.
The problem with the idea you advanced above -- which is an idea I have also advanced a few times -- is that it assumes a sort of irrationality on the part of the voter, or perhaps the better word is naivete. A state constitutional amendment, of course, does not protect abortion rights against a federal ban. So if protecting your abortion right is important, and you're in the ballot box, why would the presence of the ballot initiative affect the way you vote for president, when one of the two candidates is responsible for the necessity of the ballot initiative? Don't get me wrong: this doesn't mean the hypothesis is false, as there is plenty of irrationality in politics (as we well know). But it should be a consideration. Presumably the people who get turned out to vote because of the abortion issue have been reached by the pro-choice campaign. If so, those people would also have pointed out that Trump is anti-choice.
Another problem with the idea is the assumption that the voters mobilized by the initiative will vote liberal. An initiative that passes by 60% doesn't necessarily have 60% support among the marginal voters. It could be that the voters who only show up because of the abortion issue are split down the middle, and that an anti-choice person will be as inspired to vote no as a pro-choice person will be to vote yes.
That said, I think there is some merit in the idea (as I wouldn't have advanced it otherwise). We aren't necessarily talking about the most sophisticated group of voters. They might just go with what is directly in front of their face on the ballot, and the second-order considerations just aren't interesting to them.
Ultimately, all such debates really depend on understanding non-voters, and also the voters who get mobilized by a ballot initiative. ASAIK people don't know enough about the psychology of non-voters, because they are hard to study.