
Since what counts as a "coup" and what is sufficient grounds for a coup to have been "attempted" are potentially tricky to adjudicate, I will use my best judgement when deciding edge cases. I am open to arguments and will aim to resolve according to the the spirit of the question.
That being said, to provide some insight into what I will be looking for, I imagine that a "coup attempt" is a serious, deliberate effort to forcibly seize the federal executive branch of the United States. The effort may be successful or unsuccessful. This question resolves YES if a coup attempt is made against Donald Trump during his second term, and NO on 21 Jan 2029 if no coup attempt has occurred. This question may also resolve NO early if Donald Trump ceases to be US President and therefore it is not possible to coup him (e.g. death, resignation).
For the coup attempt to be "against" Donald Trump, the aim of the coup would be to disempower Trump - a coup aimed at empowering Trump, such as an autocoup, would not count as a coup attempt "against" Donald Trump.
examples:
credible evidence of coup plotting that was never taken anywhere would not count
use of the 25th amendment by JD Vance and Trump's cabinet would not count
demonstrations calling for the government to resign would not count; demonstrations may, however, co-occur alongside an effort that directly seizes the president/relevant institutions of power
a massive national protest that results in Donald Trump's resignation or flight from the country would not count as a coup
the assassination of Donald Trump would not count as a coup attempt unless paired with other actions that are typical of an effort to seize state power, e.g. militias/military capturing strategic buildings in Washington DC
the Wagner-style rebellion of US military personnel that march on Washington DC intent on seizing power would count, even if they fizzled out or were thwarted
People are also trading
@EvanDaniel Great question. I lean towards not counting January 6th, primarily because it seems like an autocoup if it is a coup at all. It seems like it's pretty disputed as to whether it rightly counts as a coup attempt, which also weakens the case for counting it. But even if I were to lean towards counting it as a coup attempt, it seems to me to have been aimed at empowering Trump, rather than disempowering Trump. This question is whether 'a coup attempt against Donald Trump' occurs 'during his second term', so a January 6th-like event, if it were to occur before the end of Trump's second term, would not be against him, but for him. As always, if people have strong objections to that I'm happy to hear reasonable arguments.
Please note I've added a clarification that for a coup attempt to be against Donald Trump, it's aim must be to disempower him. I think this is mostly clear from the original wording, but have added the new line for total clarity that a coup to empower Trump (such as an auto-coup) would not count as a coup "against" Trump.