Republican support of Israel and its hawkish prime minister, Benjamin Netanyahu, has been on display as President Obama nears a nuclear agreement with Iran that critics call dangerous. The House speaker, John A. Boehner, invited Mr. Netanyahu to address Congress and 47 Senate Republicans signed an open letter to Iran warning against a deal. Mr. Boehner and Senator Mitch McConnell, the Republican leader, will each be in Israel in coming days even as the Iran talks reach a possible conclusion.

Image Former Secretary of State James A. Baker III in 2014. Credit... William B. Plowman/NBC

The shift in the party’s attitude toward Israel stems from several factors, according to Republicans — a greater sense of solidarity in the fight against Islamic extremism since the Sept. 11, 2001, attacks increased support for the Jewish state among evangelical Christians and the influence of wealthy donors like Sheldon Adelson, the Las Vegas casino magnate. And the more Mr. Obama feuds with Mr. Netanyahu, the more Republicans feel motivated to come to the Israeli leader’s defense.

“It is remarkable,” said William Kristol, editor of The Weekly Standard, a conservative magazine and one of the leading voices promoting Israel’s cause in the United States. Mr. Netanyahu, who goes by the nickname Bibi, has become a rallying point for Republicans, he said. “Bibi would probably win the Republican nomination if it were legal,” he said.

Mr. Kristol, emailing from Israel where he was meeting with Mr. Netanyahu, described the shift as a result of broader underlying trends in American politics as the political left grows more “European” and the political right grows more “Reaganite.” He added that “the conservative belief in American exceptionalism is akin to Zionism.” And he said the contrast between Mr. Obama’s friction with Mr. Netanyahu and former President George W. Bush’s strong support for Israel “is pretty dramatic.”

Jeremy Ben-Ami, founder and president of J Street, the liberal pro-Israel advocacy organization that hosted Mr. Baker at its convention in Washington this week, said the Republican Party had grown more radical, leaving behind the former secretary of state and others like Brent Scowcroft, who was national security adviser under the first President George Bush, and Colin L. Powell, another former secretary of state.