Their piece begins with these two paragraphs, which nicely summarizes their argument:
"Four months ago, we tried ... Trump for abusing the power of his office in ways that undermined our country’s national security, the integrity of U.S. elections[,] and the constitutional structure of our republic. Trump’s efforts to coerce an ally to help him cheat in the upcoming election violated the public trust, went to the heart of his unfitness for office — and revealed that he prioritizes his interests over those of the nation."
"The president was not changed by impeachment. He is as lawless and corrupt as ever. But his wrongdoing has far greater consequences given the acute challenges facing the nation, the failure of those around him to curb destructive impulses, and the continued unwillingness of many members of Congress to serve as a meaningful check and balance as the Founders intended."
I'd argue Trump is even worse. He doesn't even hide any longer that he's abusing his office. He doesn't have to. He knows he can get away with it.
And arguably, if Trump is reelected, he will get worse still. He'll simply ignore Congress whenever he wants. He'll sue when they attempt to stand up to him and then drag that out endlessly with pointless appeals whilst continuing to do what he wants. And, God forbid, he may even try to extend his presidency beyond the constitutionally limited eight years.
Our last chance to stop him is on election day, November third. He must be resoundingly defeated before he damages America beyond repair.