I don't know. They got 31% of the vote in 2014 and that was a huge victory by modern Indian standards. I'd estimate atleast 20% of that came from hardcore fans- this base is unshakable and it's also the most motivated voting bloc in the country right now. But while that's a huge core base, it can't ensure them victory by itself, they got 24% in 2009 and still lost by a huge margin.

Now the extra 11-12% of the electorate that gave them the win should be a swing vote in theory but they were voting for BJP/Modi not just because they decided it's the better choice on independant merit but also because they were pissed off at Congress and their allies. I simply can't see the current opposition winning back those people even if they falls out of love with BJP. It doesn't help that apart from a few exceptions all the opposition parties fall in the same left-of-centre space with lingering anti-incumbency factors from being in power for decades... its going to be hard for any one party to stand out and the vote will break in favour of the BJP.