CAMBRIDGE — Alan Dershowitz has been a Democrat his entire adult life. He voted for Hillary Clinton. He supported Obama. He taught at Harvard Law for 50 years. He lives in Cambridge — one of the most liberal cities in America.

On Tuesday, the 87-year-old lawyer announced he's done. He's registered as a Republican.

The reason, in his telling, comes down to one thing: Israel.

"The Democratic Party has become the most anti-Israel party in modern history," he wrote. "It openly embraces antisemitism."

Dershowitz didn't mince words about what he's giving up. He says he still agrees with Democrats on abortion, immigration, separation of church and state, and taxation. He calls himself a "foreign policy Republican" — someone who would put an asterisk next to his registration if the form allowed it.

He briefly went independent after leaving the Democrats in 2024, but said that felt like "something of a cop out." As an independent, he had no influence over either party. As a Republican, he hopes to push back on positions he disagrees with from the inside.

"I will lose even more friends than I did for defending President Trump's constitutional rights," he wrote. "Already I am getting emails, texts and calls from people urging me to reconsider."

He isn't reconsidering.

"Until the Democrats gain their sanity, I will do everything in my power to try to prevent the current Democratic Party from taking control of the House, the Senate and the Presidency."

Coming from a man who lives in a city where registered Democrats outnumber Republicans roughly 10 to 1, that's not a quiet statement. Whether it moves any needles in Massachusetts is another question — but for a party trying to prove it can attract people beyond its base, a Harvard Law professor from Cambridge switching teams isn't nothing.