As Donald J. Trump weighs whether to open an unusually early White House campaign, a New York Times/Siena College poll shows that his post-presidential quest to consolidate his support within the Republican Party has instead left him weakened, with nearly half the party’s primary voters seeking someone different for president in 2024 and a significant number vowing to abandon him if he wins the nomination. By focusing on political payback inside his party instead of tending to wounds opened by his alarming attempts to cling to power after his 2020 defeat, Mr. Trump appears to have only deepened fault lines...