Over the last five years, the Jewish political advocacy group J Street reached new heights of influence. The center-left lobby, whose slogan upon its founding in 2008 was “pro-Israel, pro-peace,” saw five Democratic presidential candidates stump at its 2019 convention. It helped persuade 48 congressional Democrats to back a 2021 bill that would have pressured Israel to further a two-state solution. In 2022, J Street had its best fund-raising year to date, a spokesman confirmed.

With the pro-Israel lobbying group AIPAC increasingly identified with the right, J Street appealed to many American Jews as reasonably moderate: standing by a democratic Israel, opposing the expansion of Israeli settlements in the West Bank and supporting the coexistence of Palestinians and Israelis in two states.

Then came Oct. 7.

For months after the Hamas attack on Israel, J Street did not call for a cease-fire. In late January, it backed a “stop to the fighting,” humanitarian aid for Palestinians and an end to Hamas control of the region. Just last month, an internal J Street email said the organization would use the word “cease-fire” — and it clarified that this was a semantic step and not a change in policy.

“I’ve never hidden the fact that I want J Street to be on the 50-yard line of the American Jewish community,” Jeremy Ben-Ami, J Street’s founder and president, said in an interview.

Mr. Ben-Ami said Friday that J Street backs the Biden administration, which warned American aid would depend on Israel’s treatment of civilians, and supports a negotiated cease-fire.

But the war has raised serious concerns within J Street’s ranks about its ability to hold that middle position without being pulled apart by forces on the right and the left. Internally, some staff members have been frustrated that the group did not call for a cease-fire much earlier. They fear J Street’s delay alienated younger Americans, including Jewish ones, who are much more likely to oppose Israel’s conduct in Gaza, as the death toll soars past 32,000 and more than 100 hostages languish.