For Mr. Schumer, the politics are particularly tricky. Few in his caucus expect him to soften his tone on the negotiations, but they are also watching where he may land should the White House pressure him to rally Democrats to support the president.

“This is a very major controversy,” Mr. Rosen said. “Everybody will be watching, of course. It’s a test for him.”

Any conflict is a potential boon to congressional Republicans, who are eager to splinter Democrats over the issue of Israeli security, a fact worrisome to Jewish Democrats, who are the majority of Jewish voters but less politically influential than right-leaning organizations like Aipac.

“The Republican Party, led by a Republican Jewish coalition, has decided the only way they can get American Jewish voters is to make Israel a partisan issue,” said Greg Rosenbaum, chairman of the National Jewish Democratic Council. “And by making Israel a political issue, they undermine the bipartisan rationale for support of the Israel-U.S. relationship.”

The bill that Mr. Schumer supports — one that would ensure the right of Congress to essentially approve or disapprove the rollback of sanctions against Iran — is being viewed by many in Washington as a proxy for all senators’ views on the underlying framework for a deal. Left-leaning groups equate support for the sanctions measures as advocating war, while hawkish groups warn that failing to support them is akin to capitulating to Iran.

The issue is actually more complex. Many senators, like Tim Kaine, Democrat of Virginia, are more concerned with ensuring the prerogative of Congress’s war powers than inherently opposed to a deal.

Others, like Dianne Feinstein, Democrat of California, are deeply resentful of the way Israel’s prime minister, Benjamin Netanyahu, in effect did an end run around Mr. Obama with his speech opposing an Iran deal to a joint meeting of Congress and are supporting Mr. Obama.