From Wikipedia,

Before, during, and after Election Day, Trump and numerous Republicans attempted to subvert the [2020 election] and overturn the results, falsely alleging that there had been widespread voter fraud, and trying to influence the vote counting process in swing states. Officials in each of the 50 states stated that there was no evidence of systematic fraud or irregularities in their state. Federal agencies overseeing election security said it was "the most secure in American history." Attorney General Bill Barr concluded there was "no evidence of widespread fraud" in the election. On multiple occasions, Trump falsely declared himself the winner. The Trump campaign and its allies continued to engage in numerous attempts to overturn the results of the election by filing dozens of legal challenges in several states, most of which were dropped or dismissed by various courts, spreading conspiracy theories falsely alleging fraud, pressuring Republican state electors and legislators, and initially refusing to cooperate with the presidential transition.

Will the next US presidential election also be considered fraudulent by the losing party?

For the purpose of this question, the "losing party" is defined as the party whose presidential candidate is projected to come in second place in the electoral college vote count by both the AP and Fox News (see fine print). This question resolves positively if ANY of the following come true following the 2024 presidential election,

• The losing candidate states that they consider the election result "fraudulent" or "rigged", or some very close synonym of those words, as determined by consensus in the comment section of this question. The losing candidate must make this claim before the scheduled inauguration. If there is a dispute as to whether the candidate did indeed say a close synonym of "fraudulent" or "rigged", and therefore a dispute over whether the word choice counts for positive resolution, then the following method of resolution will be used: if thesaurus.com lists "fraud", "fraudulent", or "rigged" as a synonym of the word, and at least one moderator agrees that it is a close synonym, then it counts for positive resolution.

• The losing candidate refuses to concede two weeks following both the AP and Fox News projecting their opponent to win the election AND neither the AP or Fox News having retracted their projection in that time. For the purpose of this question, a concession is a clear statement from the candidate projected to lose that they have acknowledged their defeat, and that they accept the results of the election. The concession must not be retracted within 48 hours.

• Fewer than 20% of congressional members of the losing party believe that their candidate lost the election as determined by credible media two weeks following both the AP and Fox News projecting the opposite-party candidate to win the election AND neither the AP or Fox News having retracted their projection in that time. By comparison, on December 7th 2020, just 27 congressional Republicans acknowledge Biden’s win according to the Washington Post.

• A survey from the Morning Consult reveals that fewer than 30% of survey participants who are members of the losing party state that the election was "probably" or "definitely" free and fair in the most recent such survey following projections from both the AP and Fox News. The relevant survey for 2020 can be found here.

Otherwise, the question resolves negatively.