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Hillary Clinton pledged on Monday that she would stand unyieldingly with Israel and warned that her potential Republican rival, Donald J. Trump, would be an unreliable partner for one of America’s closest allies.

In a rock-ribbed speech in Washington that previewed how she might confront Mr. Trump on foreign policy in a general-election campaign, Mrs. Clinton said, “We need steady hands, not a president who says he’s neutral on Monday, pro-Israel on Tuesday, and who-knows-what on Wednesday.”

“America can’t ever be neutral when it comes to Israel’s security and survival,” Mrs. Clinton said, speaking to the annual policy meeting of the American Israel Public Affairs Committee, or Aipac, the nation’s most influential pro-Israel lobbying group. “My friends, Israel’s security is nonnegotiable.”

Mr. Trump has said in recent weeks that he would be “neutral” when it came to negotiating a peace accord between Israelis and Palestinians. The remark, in substance, did not stray far from traditional American policy. But his blunt language rattled some Israelis, who worry that it might mean a less supportive United States.

Mrs. Clinton wasted no time in seizing on those fears. Her speech was a thunderous affirmation of American solidarity with Israel, with promises to buttress Israel’s military, combat anti-Semitism, police Iran on its nuclear program, crack down on Iranian proxies like Hezbollah, and thwart efforts to boycott Israeli products.

“We must repudiate all efforts to malign, isolate and impugn Israel and the Jewish people,” she said.

Mrs. Clinton’s speech was the first of a hectic political day at this annual conference, which showcases Israel’s political might in Washington. Mr. Trump is scheduled to speak to the group later in the day, as are his two remaining Republican rivals, Senator Ted Cruz of Texas and Gov. John Kasich of Ohio.

The speeches from Mrs. Clinton and Mr. Trump promised to sharpen the foreign-policy contrast between them as they pivot toward a potential general election clash. Mrs. Clinton played up her credentials as a commander-in-chief and accused the Republican candidates of lacking either experience or the will to extend American leadership in the Middle East.

“We have to get this right,” she said of the election.

Referring to the Republican lineup later in the day, Mrs. Clinton said it would give a glimpse of a foreign policy that would “insult our allies” and “embolden our adversaries.”

Mrs. Clinton also declared she would allow no daylight between the United States and Israel on the peace process. In a potential rift with the White House, she said she would oppose a United Nations Security Council resolution that sought to impose the contours of a two-state solution. President Obama is contemplating such a move before he leaves office, though he may instead opt to lay down parameters for a deal in a speech.

“I would vigorously oppose an attempt by any outside party to impose a solution, including by the United Nations Security Council,” she said.

Mrs. Clinton reaffirmed her support for Mr. Obama’s nuclear deal with Iran, but she repeated her warning that she would use military force, if necessary, to punish Iran for violating the terms of the agreement.

After the speech, the Republican National Committee released a statement critical of Mrs. Clinton’s remarks and questioning her long-term commitment to Israel.

“Hillary Clinton’s speech was an attempt to whitewash a record of undermining Israel’s interests when she was secretary of state,” the statement said. “As the chief architect of the Obama administration’s foreign policy, her support for policies like the disastrous Iran nuclear deal and her boasting of being called the Obama administration’s ‘designated yeller’ for Israel calls into question her commitment to our closest ally in the world’s most tumultuous region.”

Mr. Trump’s appearance at Aipac has touched off a wave of upset among some Jewish groups. A number of rabbis and others at Aipac are planning a protest as he talks, and Jonathan A. Greenblatt, the national director of the Anti-Defamation League, wrote in a piece for Time magazine that the group was redirecting $56,000 in contributions that Mr. Trump had made to his organization over the years.

“Our history, our faith and our values teach us that we cannot sit idly by when others are singled out for derision and when intolerance is fed,” Mr. Greenblatt wrote, describing Mr. Trump’s comments about immigrants and Muslims.

Mrs. Clinton is a familiar figure at these meetings, though she does not always bear as reassuring a message as she did on Monday. In March 2010, while serving as secretary of state, she sharply criticized the Israeli authorities for approving new Jewish housing in a Jewish neighborhood of East Jerusalem at a sensitive moment, when the United States was trying to get the Israelis and Palestinians back to the negotiating table.

In that speech, which was less warmly received than Monday’s, Mrs. Clinton described the American role in the peace process as, if not neutral, than as an honest broker between the two sides. “Our credibility in this process,” she said, “depends in part on our willingness to praise both sides when they are courageous, and when we don’t agree, to say so, and say so unequivocally.”

Mrs. Clinton’s Democratic rival, Senator Bernie Sanders of Vermont, is the only presidential candidate not addressing the conference. A senior official at Aipac said Mr. Sanders had been invited to speak but could not come to Washington because of his campaign schedule. In a change of policy this year, Aipac did not allow presidential candidates to speak to the group by satellite.

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