I get what they're saying when they say 'the election is Trump's to lose'. To me it's just emphasizing that we Dems simply cannot be complacent in any way, shape, or form. Harris/Walz are going to have to be on their top game no matter how deranged Trump/Vance are. We know where complacency got us in 2016, and how we sweated the swing states until the wee hours in 2020. We 'know it', but it needs to be seared on our foreheads until this election is over.
So maybe it's also fair to say that 'the election is Harris' to win', also. But she and Walz are going to have to fight a bit harder to win it than Trump is going to have to fuck up to lose it. Both of them have their solid 'base'. What each of them needs to do is win the undecideds, the neverTrumpers, and the 'never the other Partys'.