American politics was more stable for the past half-century than it had been for most of the country’s history. Incumbent presidents often won re-election. No major political figure was assassinated. The two parties’ basic ideologies and coalitions remained similar.

But yesterday’s remarkable events — in both Washington and Milwaukee, among both Democrats and Republicans — showed how unstable our politics have become.

During the day, attention focused on the turmoil in the Democratic Party, as senior Democrats intensified their efforts to push President Biden out of the presidential race, believing that he is too visibly aged to win re-election. Biden himself has begun to accept the idea that he may have to drop out of the race, people close to him told The Times.

At night, Donald Trump took the stage to accept the Republican presidential nomination — only five days after he was nearly killed by a gunman, two months after he was convicted of 34 felony counts in New York and three and a half years after he attempted to overturn the last presidential election. Trump gave a speech that was both personal and rambling and that highlighted how different he is from any other recent president. He also demonstrated how he transformed the party over the past decade into a populist-sounding, antiwar, immigration-skeptical movement that Ronald Reagan would hardly recognize.

(Watch my colleague Maggie Haberman break down the speech in this short video.)

It remains unclear how much Trump would govern as a populist, of course. His proposed policies are a mix of actually populist (like trade restrictions) and laissez-faire (like a large tax cut for the affluent). But these questions underscore the new political uncertainty.

On the Democratic side, the short-term turmoil is greater yet. The chances that Biden will drop out have clearly surged in the past couple of days. But the outcome remains uncertain. If he does quit the race, will the party quickly support Vice President Kamala Harris as the nominee? Or will the situation be more typical of historical conventions when the nominee was unclear, with a messy, competitive process in which multiple candidates make their cases to delegates?

President Biden in Nevada this week. Eric Lee/The New York Times

Regardless, it now seems likely Biden will not be the person who takes the oath of office on Jan. 20, 2025. That would mean that two consecutive presidents — Trump in 2020, Biden in 2024 — left office before they wanted to, when they were eligible to serve another four years. The last time the country experienced such back-to-back presidencies was more than 40 years ago, with Gerald Ford and Jimmy Carter, during the chaotic years of Watergate and oil crises.

In the rest of today’s newsletter, we’ll highlight the key moments from Trump’s speech and update you on the latest Biden developments.