If you look at 'do you want to introduce new grammars?' against maintaining the status quo or abolishing them, then I don't think the data supports them
A recent poll asked, "Would you support or oppose re-introducing grammar schools across the whole of Great Britain". This found over 50% in favour with 20% against. The rest were don't knows. Supporters outnumbered opponents in every political and demographic group.
An alternative question, "Would you support or oppose re-introducing the selective education system across the whole of Great Britain, where children take an exam at 11, with the top quarter of children going to grammar schools and the other three-quarters going to secondary modern schools", comes up with a narrower margin but those expressing an opinion still break heavily in favour of grammar schools with roughly 4 in favour for every 3 against.
And I seem to recall reading that only 30% of parents of school-aged kids are in favour of new grammars
I have not seen a poll showing support anywhere near that low. Polls I have seen suggest that around two thirds of parents would get their children to sit the entrance exam and would send them to grammar school if they passed.
YouGov recently did an analysis by local authority area and produced a map of England showing levels of support. This again showed that parents in most LAs are in favour of grammar schools.
I am not personally in favour of grammar schools as they were when I went to school. I want to see how these proposals differ before I make any judgements. But all the evidence is that the policy is popular with voters. That may change, of course, but right now the policy does not appear to be a vote loser.