Mr. Trump also is expected to announce the shifting of the United States Embassy from Tel Aviv to Jerusalem. Critics say that the announcements would be the death knell for an already moribund peace process and would pull the plug on a two-state solution.
The change in policy would, they say, remove any pretense of the United States as a neutral broker for peace, and Palestinians and other Arabs in the region would view the Trump administration as extension of Israel’s right-wing government.
The change in American policy “destroys the peace process,” said the Palestinian prime minister, Rami Hamdallah.
Some of the most traditional allies of the United States expressed apprehension.
“Clearly this is a decision that makes it more important than ever that the long-awaited American proposals on the Middle East peace process are now brought forward,” Britain’s foreign minister, Boris Johnson, told reporters in Brussels on Wednesday.
That process, led by Mr. Trump’s son-in-law, Jared Kushner, has seemingly failed to get off the ground.
Leaders in the region have already warned against the move. A statement from the royal palace of King Abdullah II of Jordan, whose kingdom is the custodian of Al Aqsa Mosque in Jerusalem, stressed that the city was a key component to “achieving peace and stability in the region and the world.”
In Turkey, President Recep Tayyip Erdogan was convening a summit meeting of the main Pan-Islamic body next week in Istanbul to discuss the American moves and to show, as his spokesman Ibrahim Kalin told reporters in Ankara, “joint action among Islamic countries.”
Mr. Kalin called the expected change a “grave mistake,” adding that “Jerusalem is our honor, Jerusalem is our common cause, Jerusalem is our red line.”
Iran, unsurprisingly, condemned the change, with the supreme leader, Ayatollah Ali Khamenei, saying at a conference in Tehran on Wednesday that it reflected the “incompetence and failure” of the American government.
Even Israel was bracing for a reaction to Mr. Trump’s expected declaration, though Mr. Trump had long telegraphed his intentions.
During the presidential campaign, Mr. Trump promised evangelical members of his base and Orthodox Jewish supporters who, unlike many other Jews, tend to vote Republican, that “we will move the American Embassy to the eternal capital of the Jewish people, Jerusalem.”
The Vatican, like Italy, has long been sympathetic to the plight of Palestinians. (Alessandro Di Battista, a leader of Italy’s Five Star Movement, called the idea of moving the embassy “a stupid provocation.”)
The Vatican established full diplomatic relations with Israel in 1994, and Pope John Paul II and Pope Benedict XVI both visited Israel and the Palestinian territories.
In 2012, the Vatican called for “an internationally guaranteed special statute” for Jerusalem, with the goal of “safeguarding the freedom of religion and of conscience, the identity and sacred character of Jerusalem as a Holy City, (and) respect for, and freedom of, access to its holy places.”
Francis visited the Holy Land in 2014, but he upset some Israelis by flying via helicopter directly from Jordan to the “State of Palestine,” as the Vatican schedule at the time referred to the territories. He visited Israel afterward.
In 2015, the Vatican entered into a treaty with the “State of Palestine.”
While the notion that conflict in Jerusalem is linked to unrest and instability throughout the region has long been disputed by Middle East analysts, the pope, along with others around the globe, seemed to consider Mr. Trump’s expected change of policy a purely political and needless aggravation.
The speech is just as likely to cause consternation among Jewish voters in the United States, who are increasingly polarized.
Jews traditionally vote in overwhelming numbers for Democrats, but the hard-right policies of Prime Minister Benjamin Netanyahu of Israel have anguished liberal Zionists in the United States, who often have found themselves torn between their liberalism and Zionism.
Mr. Trump’s declaration is likely to add to the radicalization of a small, but increasing number of American Jews who, already disgusted by the Netanyahu government, have promoted boycotts and sanctions against Israel, which they publicly call a racist or apartheid state.
The hostility of the growing left-wing fringe of American Jews is seeping into the Democratic Party, where, in 2012, delegates to the Democratic National Convention booed when officials sought to reinstate a recognition of Jerusalem as the capital of Israel into the party platform.
But the change from a party platform to definitive American foreign policy is a major one that has seized the world’s attention.
On Tuesday, Francis spoke by telephone to the Palestinian Authority president, Mahmoud Abbas, about the unfolding crisis. And before the pope’s public remarks to the faithful at the Vatican on Wednesday, he met privately with a group of Palestinians participating in interfaith dialogue with officials at the Vatican.
“The Holy Land is for us Christians the land par excellence of dialogue between God and mankind,” he said. “The primary condition of that dialogue is reciprocal respect and a commitment to strengthening that respect, for the sake of recognizing the rights of all people, wherever they happen to be.”