Rep. Alexandria Ocasio-Cortez, D-N.Y., and the powerful American Israel Public Affairs Committee (AIPAC) are facing off over a letter the progressive firebrand sent Tuesday warning against Israeli efforts to annex parts of the West Bank.

AIPAC took the rare step of warning lawmakers against signing onto Ocasio-Cortez’s letter to Secretary of State Mike Pompeo, using a Twitter thread Monday to argue that the letter “explicitly threatens the US-Israel relationship.” Eleven other House Democrats and failed presidential candidate Sen. Bernie Sanders, I-Vt., nonetheless jointed Ocasio-Cortez and signed onto the letter.

“Should the Israeli government move forward with these plans, they would actively harm prospects for a future in which all Israelis and Palestinians can live with full equality, human rights and dignity, and would lay the groundwork for Israel becoming an apartheid state, as your predecessor John Kerry warned in 2014,” the 13 lawmakers wrote Pompeo.

Why it matters: AIPAC’s public rebuke of the Ocasio-Cortez letter followed a quieter yet unsuccessful attempt to dissuade the majority of House Democrats from signing onto a separate letter warning the Israelis against annexation. Several AIPAC-friendly Democrats, including House Majority Leader Steny Hoyer, D-Md., Middle East panel Chairman Ted Deutch, D-Fla., and Rep. Brad Schneider, D-Ill., signed onto last week’s letter.