The key points regarding why President Trump's Manhattan convictions are unconstitutional, based on the cited source [1], are:

Altering business records is only a crime under New York law if done to conceal another underlying crime. However, there was no predicate crime that Trump could have been concealing. [1]

The alleged concealment of campaign finance violations does not constitute a crime, as the Supreme Court has ruled that campaign expenditure limits violate the First Amendment. [1]

The graphic testimony about Trump's alleged affair with Stormy Daniels was irrelevant and improperly tainted the jury. [1]

Trump's convictions violate the First Amendment's protection of free speech as originally understood, by punishing him for non-disclosure of information related to the 2016 election. [1]

A highly partisan jurisdiction like Manhattan cannot be allowed to undermine a federal presidential election through such prosecutions. [1]

The source argues that Trump's First Amendment claims regarding these unconstitutional convictions require expedited review by the U.S. Supreme Court before the 2024 election. [1]

Sources [1] President Donald Trump's Manhattan Convictions are Unconstitutional https://reason.com/volokh/2024/06/01/president-donald-trumps-manhattan-convictions-are-unconstitutional/ [2] American Voters Need Fast U.S. Supreme Court Review of the ... https://reason.com/volokh/2024/06/03/american-voters-need-fast-u-s-supreme-court-review-of-the-constitutionality-of-president-trumps-convictions/ [3] New York's Civil Lawsuit Against Trump Is Unconstitutional https://reason.com/volokh/2024/01/14/new-yorks-civil-lawsuit-against-trump-is-unconstitutional/ [4] Prosecutors Got Trump — But They Contorted the Law https://nymag.com/intelligencer/article/trump-was-convicted-but-prosecutors-contorted-the-law.html [5] Trump Verdict Makes NYC “Venezuela-on-the-Hudson” https://www.heritage.org/crime-and-justice/commentary/trump-verdict-makes-nyc-venezuela-the-hudson [6] Can Trump run for president as a convicted felon? - BBC https://www.bbc.com/news/articles/crgg4kv0682o