// Until he's convicted of committing a crime I can't see how he can be deemed ineligible. //
This seems to be an ongoing legal argument, see eg
https://archive.ph/20230820122539/https://www.theatlantic.com/ideas/archive/2023/08/donald-trump-constitutionally-prohibited-presidency/675048/ and
https://papers.ssrn.com/sol3/papers.cfm?abstract_id=4532751 . I have no comment on the correctness of the arguments, but I've quoted a couple of key passages below.
Firstly, the relevant Constitutional Amendment (14.3), lightly edited to remove irrelevant alternative cases:
// No person shall ... hold any office, civil or military, under the United States, who, having previously taken an oath ... to support the Constitution of the United States, shall have engaged in insurrection or rebellion against the same... //
Secondly, some quotes from the AtlanticToday article:
// Having thought long and deeply about the text, history, and purpose of the Fourteenth Amendment’s disqualification clause for much of our professional careers, both of us concluded some years ago that, in fact, a conviction would be beside the point. The disqualification clause operates independently of any such criminal proceedings ... [and] was designed to operate directly and immediately upon those who betray their oaths to the Constitution, whether by taking up arms to overturn our government or by waging war on our government by attempting to overturn a presidential election through a bloodless coup. //
// The former president’s efforts to overturn the 2020 presidential ... place him squarely within the ambit of the disqualification clause, and he is therefore ineligible to serve as president ever again. The most pressing constitutional question facing our country at this moment, then, is whether we will abide by this clear command of the Fourteenth Amendment’s disqualification clause. //
It goes on, but the only other point I want to make now is that the scholarly article above is written by "two of the most prominent conservative constitutional scholars in America", which if true would at least somewhat shield them from accusations of political bias.
It's all a mess. For my part, I have to say that even if the argument were academically correct, it might be politically extremely difficult to defend, especially without a criminal conviction. On the other hand, the wheels of justice are moving very slowly indeed, and it's extremely unlikely that the various relevant trials will be resolved prior to the election.