I am not persuaded that the ability to have huge rallies is a good measure of the level of a candidate’s support. It also depends on how good the campaign staff and the advance teams are at drawing in a crowd. Furthermore, if a candidate has a core base of fervent supporters, they may be able to pack rally after rally by following him from place to place, like Grateful Dead fans, even while broader support may be declining.
But there is no question that having a huge rally provides a big psychological boost to the candidate. Having a large crowd turn up to cheer you on can undoubtedly be energizing. For a narcissist and egomaniac like Trump, having rallies is particularly important which is why the lackluster turnouts at Tulsa, OK and the airport tarmac in Tampa, FL must have been devastating. It increasingly looks like the cancelled rally in Portsmouth, NH was very likely due to fears of low attendance. And now the Republican convention is also going to be a modest affair, which may be why the Trump campaign reportedly wants to ban media access to the event, ostensibly because of coronavirus restrictions but more likely due to fearing that he would not benefit from being seen speaking to a small crowd that is forced to be spread out.
But while we can be sure that Trump really needs a crowd for his ego, I suspect that his supporters may need them too, perhaps even more than him. The members of the kind of cult following that Trump has cultivated need to have their beliefs regularly affirmed and the way that cults do this is by gathering people together to repeatedly and enthusiastically affirm their loyalty in a group setting. They may need the big rallies to reassure themselves that the polls showing a decline in Trump’s support are not valid and have somehow been rigged against him. If people are alone and isolated, fervor can die and faith can waver. This is partly why, I think, some churches are so adamant in demanding that, against the best medical advice, they be allowed to have physical church services even during the pandemic. It better enables them to bolster the faith of their congregation, not to mention enabling the better collection of money.
So the worst affected by the absence of large rallies may not be Trump but his followers.