It is safe to say that the first 100 days of Donald Trump’s second presidency will be considered the most consequential of any in modern history. Since taking office, Mr. Trump has consolidated extraordinary power in the executive branch, dismantled large portions of the federal government, undone the military and economic alliances that were formed following World War II and torn up the policy consensus that has governed global trade for just as long.

But a consequential start does not in any way equate to long-term success. Mr. Trump’s approval ratings are already falling, and if past presidencies are any guide, the worst is yet to come. Now as in 2017, he is the only modern president to have a net negative approval rating at this point in his term. And as more and more Americans begin to feel the pain of his policies, we may well look back on his first 100 days as the prelude to a historically unpopular presidency.

Time has shown that the president typically enjoys a honeymoon period after the election, during which his approval rating is inflated. A more accurate picture of his popularity tends to emerge by early September of his first year, after the public has had time to absorb the impact of the new administration’s agenda. In every case since the 1990s, the job approval of a new president was measurably lower at that point than it was at the 100-day mark.

By the first September of their first terms, Bill Clinton and George W. Bush had suffered an 11-point decline in their job approval. (Mr. Bush’s downward trajectory was disrupted by Sept. 11, 2001, which essentially made him a wartime president.) For Barack Obama, the drop was 15 points. For Mr. Trump, it was five points.

Joe Biden, the only one of these presidents not elected to a second term, had a 57 percent job approval rating as he approached his 100th day in office. By early September, however, it was down to 43 percent, in large part because of his premature claims of Covid’s defeat, the botched withdrawal of U.S. troops from Afghanistan and a surge in inflation. Growing concerns about his fitness for office ensured his job approval never recovered, forcing him to withdraw from the general election 107 days before Election Day.