Can a U.S. presidential candidate successfully run for national office without being on the ballot in one or more states? For example, recently some U.S. states have adopted requirements for presidential candidates' disclosure of tax returns. If a presidential candidate did not want to comply with that requirement, they would not appear on the ballot in that state. Assuming the candidate won the electoral college by virtue of the other states, is there any reason they would not become the president-elect?
Can a U.S. presidential candidate successfully run for national office without being on the ballot in one or more states?
Yes, they can.
In the 2016 presidential election, many minor candidates were not on the ballot in all states. 31 people were on the ballot in at least one state, of which only three appeared on all ballots (Those three are the Republican, Democratic, and Libertarian party candidates).
For example, the Green Party candidate had, according to Wikipedia:
Ballot access to 480 electoral votes (522 with write-in) [out of 538]:
- As write-in: Georgia, Indiana, North Carolina
- Ballot access lawsuit pending: Oklahoma
- No ballot access: Nevada, South Dakota
So it's not a requirement to be on all ballots to participate as a candidate.
Of course, being elected requires acquiring a majority of the Electoral Votes. This will be harder if you don't fight for every electoral vote. Giving up on some electoral votes while all it takes is some locals gathering some signatures and the party paying a "small" amount of money, doesn't seem wise.
On the other hand, there are states that are so overwhelmingly in favor of one party, that the other party might as well not be on the ballot. However, ignoring one's voters in a full state might not be taken well by one's potential voters in other states. And you need to not alienate those voters in swing states.
And as @Delioth pointed out in comments, there are many elections on the same day in the USA. Even though the presidential candidate might not win this state, his or her name still draws additional voters to the ballot box. Same party candidates for other elections might benefit from the additional votes cast by those additional voters.
Therefore in practice, I expect the major candidates to be on the ballot in all states. But it's not a legal requirement.