WASHINGTON — As the proposed agreement over Iran’s nuclear program is debated in coming weeks, President Obama will make his case to a Congress controlled by Republicans who are more fervently pro-Israel than ever, partly a result of ideology, but also a product of a surge in donations and campaign spending on their behalf by a small group of wealthy donors.

One of the surprisingly high-profile critics is Senator Tom Cotton of Arkansas, who burst to prominence with a letter signed by 46 Republican colleagues to leaders of Iran warning against a deal. Mr. Cotton, echoing criticism by Israeli leaders, swiftly denounced the framework reached on Thursday as “a list of dangerous U.S. concessions that will put Iran on the path to nuclear weapons” — words, his colleagues say, that expressed his deep concern about Iran’s threat to Israel’s security.

But it is also true that Mr. Cotton and other Republicans benefited from millions in campaign spending in 2014 by several pro-Israel Republican billionaires and other influential American donors who helped them topple Democratic opponents.

Republicans currently in the Senate raised more money during the 2014 election cycle in direct, federally regulated campaign contributions from individuals and political action committees deemed pro-Israel than their Democratic counterparts, according to data compiled by the Center for Responsive Politics and analyzed for The New York Times by a second nonprofit, MapLight. The Republican advantage was the first in more than a decade.