I declare this blog post to be interesting

Donald Trump lost the US election last week to Joe Biden. You might have missed it, it's not like it was literally all anyone was talking about all week. We even stopped talking about coronavirus for a bit. One of the things that characterised his unhinged campaign was an insistence on the election being fraudulent, and also that he had definitely got more votes. Because of the way the system works over there, states are 'called' for one or the other candidate once it's past reasonable doubt about who will win (e.g. the number of votes still to count isn't enough to change the current outcome). This means that anyone could call a state early, and they might be right, because it's 50/50, but they might also be wrong because they're just guessing, so you want to look at the basis on which they're calling it. 

Trump tweeted this, and Twitter immediately marked it as false or misleading: 

We have claimed, for Electoral Vote purposes, the Commonwealth of Pennsylvania (which won’t allow legal observers) the State of Georgia, and the State of North Carolina, each one of which has a BIG Trump lead. Additionally, we hereby claim the State of Michigan if, in fact,.....

— Donald J. Trump (@realDonaldTrump) November 4, 2020

You can see he uses the word hereby in his claim for Michigan, which adds an air of authority and legality to the claim, although no actual legality of course. 

Hereby is a word meaning 'as a result of this utterance', so if you say I hereby declare this play park open, then you have opened the play park by uttering those words. You can also, if invested with the right authority, hereby forbid barbecues in this area before 6pm. This is what's known as a 'performative' speech act, because by uttering the thing, you are performing that act: claiming, declaring, promising, naming, and so on. Hereby is useful for legal documents, because it also means 'as a result of this document', so it means something like 'by signing this document you agree to this rule', and also that if the document is later overturned, so too is the thing that was hereby excluded/forbidden/claimed. 

In everyday speech it's actually less useful because the verb does most of the work, and the legality or validity is provided by the situation. If you don't have the right authority, you can of course still declare it, forbid it, whatever, but it doesn't have legal standing. So if you stand in your neighbour's garden and hereby declare that this is your property, it doesn't magically become your property. However, you have still declared it, and you would have even if you hadn't said hereby. There is no difference between 1 and 2 except for some posturing, and perhaps an implication that you're just now claiming it in (1), whereas in (2) perhaps there is some prior claim on it that you're bringing up now (it was promised to you by the host earlier on, perhaps). Your claim being upheld is dependent on the agreement of your friends. Similarly, there's no difference between (3) and (4), and the validity of your declaration is dependent only on you being some local dignity who's been asked to officially open the shopping centre; it isn't open if you just pass by a half-built complex and shout this at your friend. 

1. I hereby claim this last slice of pizza. 
2. I claim this last slice of pizza.
3. I hereby declare this shopping centre open. 
4. I declare this shopping centre open.

In a sentence like the one in my post's title, where I declare something subjective, then the validity is dependent on people agreeing with me. This is why statements like this are better if they declare something objective, but are also used to give additional (sometimes false) weight to opinions or baseless claims. My post might be dull as anything, or only some people might agree, but me declaring it to be interesting sounds like I have some authority to make a factual statement about it. 

So Trump claims Georgia and North Carolina just as much as he does Michigan, even though he doesn't say hereby for those ones, and the speech act is the same. The legality or validity of it is dependent not on his claiming them, but on the number of votes counted. At the time of writing, the former two are still not called by Associated Press, and of course his claim for Michigan came with a big censored 'if' clause about voter fraud, so no doubt he will be pursuing that one.