As President Biden visited Israel on Wednesday to pledge U.S. support in its fight against Hamas, his nominee to fill the conspicuously empty American ambassador’s post to the country faced Republican opposition at home.

The nomination of Jacob J. Lew, who served as Treasury secretary during the Obama administration, was announced last month but has gained urgency since Hamas attackers slaughtered at least 1,400 Israelis on Oct. 7. The attacks prompted Israel to launch an intensive bombing campaign against the Gaza Strip, which is governed by Hamas.

But despite pleas from the Biden administration and leading Democrats to confirm Mr. Lew quickly, most Republican senators on the Foreign Relations Committee expressed opposition to his confirmation, arguing he was unfit for the post because of his past work on a multinational deal to curb Iran’s nuclear ambitions.

“We need this thing filled. The problem I have is that it needs to be filled with the right person,” Senator Jim Risch of Idaho, the top Republican on the panel, said during Mr. Lew’s confirmation hearing Wednesday. He accused Mr. Lew of acting in “what I think was a backhanded way of supporting Iran.”

As Treasury secretary, Mr. Lew played a key role in negotiating and promoting a deal that eased sanctions on Iran’s nuclear sector in exchange for Tehran’s accepting limitations on its nuclear enrichment and related activities. But Republicans charge that Mr. Lew lied to lawmakers at the time about how the deal would be implemented, particularly when it came to access to previously frozen funds to Iran, which they believe Iran used to build up terrorist proxy groups posing a threat to Israel, including Hamas.

The Republican opposition is not expected to kill Mr. Lew’s candidacy. Democrats — who expressed support for him — hold a majority on the panel. But it could complicate efforts to get his bid through the full Senate quickly, by forcing Senator Chuck Schumer, the majority leader, to use time-consuming procedural maneuvers to set up a vote.

Israel has lacked a Senate-confirmed U.S. ambassador since Thomas R. Nides left the post in July. Mr. Schumer and other Democrats have warned that continuing to leave the post empty would be detrimental to the United States’ ability to support Israel.

“Delaying him would be egregious at a time like this,” Mr. Schumer said Tuesday of Mr. Lew on the floor, adding that the nominee was “a ferocious ally of Israel.”

Mr. Lew sought to display his pro-Israel bona fides on Wednesday, telling senators about his Jewish family’s Zionist roots and listing his efforts as Treasury secretary to impose and enforce sanctions against both Hamas and Iran.

“I cannot remember a time when Israel’s struggle for security was not at the forefront of my mind,” Mr. Lew said. He added: “Iran is a threat to regional stability and to Israel’s existence. If confirmed, I will uphold President Biden’s commitment to deny Iran a nuclear weapon.”

But his declarations did little to deter Republican senators.

Senator Marco Rubio, Republican of Florida, accused Mr. Lew of having kept secret from Congress a license that would have allowed Iran to convert funds held at a bank in Oman into euros, and pressured banks around the world to do business with Tehran, citing the findings from a 2018 report from Republicans on the Senate Permanent Subcommittee on Investigations. Senator John Barrasso, Republican of Wyoming, accused Mr. Lew of acting as the Iranian government’s banker by negotiating deals to release funds in exchange for American prisoners.

“The massive influx of cash was ultimately a direct deposit into Iran’s terrorism account,” Mr. Barrasso said.

Mr. Lew argued that he had adhered strictly to the terms of the deal easing nuclear-related sanctions, known as the Joint Comprehensive Plan of Action — and frustrated officials in Tehran by refusing to go any further.

“They complained that my actions were what kept them from getting full access to the world financial system,” he said. “We did the letter of the J.C.P.O.A., nothing more.”

If confirmed as ambassador, Mr. Lew will be starting at a time of heightened fears that the conflict between Israel and Hamas might engulf the region, potentially even pulling in U.S. forces.

“Preventing this from becoming a multifront war is hugely important for Israel, for the region, for the world,” he said. He added that bringing home the Americans thought to be hostages of Hamas would be one of his top priorities. At least 13 Americans are unaccounted for, according to the State Department, but it is unclear how many are being held hostage.

Yet Mr. Lew conceded that longer-term goals such as the normalization of relations between Israel and Saudi Arabia, and an eventual two-state solution with the Palestinians, would probably have to wait until hostilities died down.

He also promised that he would do his utmost to ensure that civilians in Gaza receive humanitarian assistance, drawing a distinction between them and Hamas militants.

But he resisted Democrats’ efforts to get him to urge the Israelis to exercise restraint.

“This is not the time for us to be lecturing Israel on what they need to do,” Mr. Lew said, adding that Israeli officials knew they had to conduct the war “in a way that is consistent with minimizing the impact on innocent civilians.”