I really don’t think that’s the play here.

1. He’s very unlikely to get a formal declaration of war out of Congress in the first place. More likely, they’ll be sneaking this action through by legal chicanery that basically requires some kind of “Special Military Operation” nonsense.
2. There is an absolutely ungodly amount of precedent for the US continuing to hold elections while at war. Courts would straight up laugh at the proposal that war with Iran is sufficient reason to cancel elections, but WW2 wasn’t.

Yes, Trump wants to cancel the mid-terms. We know because he’s told us, several times. No one is arguing with that. But at some point Trump’s plans do go through the phase where he tells a bunch of people to make it happen, and there’s no question that those people told him this angle isn’t going to work.

Look no further than their pivot to “Taking over” elections instead of cancelling them. This is the usual result of Trump’s bad ideas. A room full of horrendously corrupt people basically come back and either say “Yeah we can probably get away with that despite how horrendously illegal it would be,” or “No, there’s no way for us to get away with that, but here’s a horrendously illegal / immoral / corrupt thing we can try to do instead.” This one was clearly the latter.

The more obvious play here is to hope for a war president bump, like Bush got. Generally Americans don’t like to vote against war presidents. But for Trump that’s far less likely to work, because he specifically made a point of running as an anti-war president. That wouldn’t matter if it was only the usual right wing bullshit that they can just discard when it’s inconvenient, but in this case Trump was actually tapping into a deeper well of anti-war sentiment that was already there, as part of a broader move towards isolationism that’s happening in the US right now. That’s why all his fascist expand the borders BS is going over so badly, even with MAGA. They don’t want Greenland, they want better jobs and cheaper groceries.

Of course, the other potential benefit to a protracted war in Iran is that it eats a lot of headline space, which is less room for the Epstein files, so that’s another possible motivation. And it can easily be both.

There’s no actual rules like that, nor is “martial law” like a bell you ring to cancel elections, but enough people believe it works this way that it’s very likely they will attempt to use this card.