It is a truism of liberal media that “Trump falsely claimed that the 2020 Presidential election was stolen”. This is probably repeated more often than any other single “true fact” in the world.
I’m interested. I know that Trump claims that the election was “stolen” – but on what basis does he claim this? I assumed he must have some basis for this claim. I did some internet searching for something like “on what basis does Trump claim that the 2020 election was stolen”. This was the first surprise. It is amazingly hard to actually find reports of Trump making specific claims. Either he didn’t make many or they are not being reported. I found a few pages which have reports like this:
TRUMP, on the Biden administration: “”That’s what you get when you have a rigged Election.”
TRUMP: “In actuality, the Big Lie was the Election itself.”https://www.pbs.org/newshour/politics/ap-fact-check-on-jan-6-anniversary-trump-sticks-to-election-falsehoods
I found a single reference where Trump referred to “illegal voting”.
Assuming that the media is not supressing a detailed accusation from Trump what we appear to have is largely a political claim. By “political” I mean a slogan designed to win elections. Something like “leaving the EU will be good for the NHS (National Health Service)”. Such a claim cannot be a “lie” or “false”. Slogans can be dishonest or an attempt to manipulate people but they don’t claim to be facts and therefore can’t be “false”. The first point here then is that the incessant discourse about “Trump’s false claims” is an egregious example of a new and very disturbing epistemological trend. In this new world of discourse there is only one kind of knowledge, the type which can be reduced to simple facts, and the more fraught and difficult world of opinions, interpretations, and views is simply cancelled. Facts can be only true or false; and only one version is, necessarily, true. This is precisely the way that “truth” was handled in the Soviet Union. Trump is not making a truth-claim; he is a politician with a slogan, but the liberal media, rather than engaging in the cut-and-thrust of debate are just trying to cancel him.
If the liberal media did engage in debate with Trump on this point they would need to counter the claim of “illegal” or “rigged” voting. As far as I can see Trump is referring in part to the matter of postal voting. The following appear to be some simple facts about the 2020 US Presidential election and postal voting:
There was a massive increase in postal voting. From 25% in elections in 2016 and 2018 to 43% in 2020. 
Democrats are much more likely to use postal votes. According to this research centre twice as many Biden voters voted by post than Trump voters. 
5 states at the time of the 2020 election were sending unsolicited postal ballot forms to every voter!  10 states now have this policy. 
Methods of verifying identity for postal voting vary. Checking the signature against a record in a database seems to be common. But in many cases district election authorities make no checks at all. It is enough that the voter has signed a declaration. 
Link  is interesting. I am quoting from a liberal media source. Notice how they try to spin this: “Election officials note that, by registering to vote, people are effectively requesting a ballot, so it makes no sense to call the materials sent to them “unsolicited.”” – Which is simply sophistry. The postal ballots are precisely unsolicited.
In other words; postal voting is favoured by Democrat voters. The 2020 Presidential election saw a massive increase in postal voting. Checks on the veracity of postal votes are various and not robust across the board.
I would say that, given that all he is doing is producing a political statement or slogan, Trump is obviously entitled to say that “the election was stolen”.
The Big Lie is indeed the Big Lie.