In the weeks before Senator Chuck Schumer’s decision to oppose the Iran nuclear accord, John Shapiro, a New York financier and a leader of the American Jewish Committee, had his ear, plying him with reasons to oppose it.

Mr. Shapiro, a longtime benefactor of the New York senator and other Democrats, was hardly alone. Some of the wealthiest and most powerful donors in American politics, those for and against the accord, tried to get a word in with Mr. Schumer. Now, approaching a vote on President Obama’s most important international priority, the fight is expanding, with tens of millions of dollars flowing into ad campaigns, and contributors leveraging access to undecided Democrats.

S. Daniel Abraham, a Jewish philanthropist who made billions on SlimFast drinks, dispatched emails to Senate Democrats while vacationing in St.-Tropez, urging their support. The gay rights patron Tim Gill and his husband, Scott Miller, also have quietly urged lawmakers to back the president, telling them the deal is in Israel’s best interest. Norman Lear, the TV producer who has raised millions for Democrats, said he wrote to Mr. Schumer to criticize his decision.

“At the very least, it was a mistake,” Mr. Lear said.

The furious lobbying lays bare the volatile politics of the Iran accord, which has already pitted Mr. Schumer, the likely future leader of Senate Democrats, against the Democratic president. It also reveals donors’ divisions over the deal — and the extraordinary access those donors have wielded to speak directly to lawmakers and their top aides.