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 among Republicans during his yearlong revenge tour. A clear majority of primary voters under 35 years old, 64 percent, as well as 65 percent of those with at least a college degree — a leading indicator of political preferences inside the donor class — told pollsters they would vote against Mr. Trump in a presidential primary.

Mr. Trump’s conduct on Jan. 6, 2021, appears to have contributed to the decline in his standing, including among a small but important segment of Republicans who could form the base of his opposition in a potential primary contest. While 75 percent of primary voters said Mr. Trump was “just exercising his right to contest the election,” nearly one in five said he “went so far that he threatened American democracy.”