WASHINGTON — Since winning election to the U.S. Senate last fall, John Fetterman has planted himself among the most liberal members in the Democratic caucus.

He joined Sen. Elizabeth Warren, D-Mass., in opposing legislation suspending the debt ceiling because it was tied to new work requirements for food stamps and other spending cuts. He endorsed two candidates for Philadelphia City Council from the liberal Working Families Party, in defiance of the local party establishment. Years earlier, he backed Sen. Bernie Sanders, I-Vt., for president over Hillary Clinton.

But ever since Hamas — designated as a terrorist group by the U.S. — attacked Israel last month, Mr. Fetterman has found himself on the opposite side of many of his progressive allies.

In the weeks after the Oct. 7 attack against Israel, which left 1,400 dead and more than 200 held hostage, the freshman Democrat repeatedly has affirmed unwavering support for the Jewish state and bucked progressives’ calls for a ceasefire. He has stood firm even after Israeli retaliatory strikes in Gaza killed thousands of Palestinians, forced even more to leave their homes, and raised calls for a ceasefire to a fever pitch.