"We will keep radical Islamic terrorists the hell out of our country," Trump told the audience attending the annual summit of the Republican Jewish Coalition.
"You remember the travel ban? On day one I will restore our travel ban."
At the start of his presidency in 2017, Trump imposed sweeping restrictions on the entry of travelers from Iran, Libya, Somalia, Syria, Yemen and, initially, Iraq and Sudan.
The order was quickly challenged in court as discriminatory against a religious group, but the bans, along with Trump's hard-line anti-immigration agenda, were popular with his base.
President Joe Biden reversed the ban in his first week in office in 2021.
Biden "was proud to overturn the vile, un-American Muslim ban enacted by his predecessor," a White House spokesman said.
The former US leader was among several Republican hopefuls lining up at the gathering of influential Jewish donors to pledge unwavering support for Israel in its war against Hamas.
Trump told the event, held in Las Vegas, in the southwestern state of Nevada, that he would "defend our friend and ally in the State of Israel like nobody has ever."
The conflict between Israel and Hamas is "a fight between civilization and savagery, between decency and depravity, and between good and evil," said Trump, who received the warmest response from attendees, as he took aim at the Biden administration and avoided criticizing his rivals.
The former reality show host, the overwhelming favorite to win the party nomination to run against Biden next year despite facing multiple criminal prosecutions, spoke after sparking fury in recent weeks by describing Lebanon-based Islamist group Hezbollah as "very smart" and criticizing Israeli Prime Minister Benjamin Netanyahu.
- Pence drops out -
Also on hand in Las Vegas was Trump's nearest rival, Florida Governor Ron DeSantis, who called the October 7 Hamas surprise attack on Israel "the most deadly attack against Jews since the Holocaust itself."
Hamas militants killed at least 1,400 people, mostly civilians.
More than 8,000 Palestinians have been killed by Israel's relentless retaliatory bombardments, mainly civilians and many of them children, according to the health ministry in the Hamas-run Gaza Strip.
DeSantis and others pointed to what they said was rising anti-Semitism on US college campuses, and proposed yanking funding for universities and canceling visas for pro-Palestinian foreign students.
"We need cultural chemotherapy to fight this cancer," Senator Tim Scott said.
"Any student with a visa who calls for genocide should be deported."
FILE - US Republican presidential candidate Tim Scott speaks at the RJC Annual Leadership Summmit on October 28, 2023 in Las Vegas, Nevada. (Photo by Frederic J. BROWN / AFP)
House Majority Leader Steve Scalise said "next week we are going to have a resolution on the floor to condemn the actions of anti-Semitism at campuses across our country".
"If you cannot stand against terrorism today, when can you?"
He mentioned several upcoming bills, including one to "give Israel the funding it needs to replenish" its strategic missile systems.
The only woman in the race, Nikki Haley, Trump's former ambassador to the United Nations, has evoked fears of anti-Semitic attacks on US soil.
Haley has vowed to "change the official federal definition of anti-Semitism to include denying Israel's right to exist," adding she would strip tax breaks from schools that don't combat anti-Semitism.
FILE - US Republican presidential candidate Nikki Haley addresses the Republican Jewish Coalition (RJC) Annual Leadership Summmit on October 28, 2023 at the Venetian Conference Center in Las Vegas, Nevada. (Photo by Frederic J. BROWN / AFP)
"College campuses are allowed to have free speech, but they are not free to spread hate that supports terrorism," she said. "Federal law requires schools to combat anti-Semitism. We will give this law teeth," she said.
The newly installed speaker of the House of Representatives, Mike Johnson, in his first address to a public event since taking on the new role, said Republicans will "stand united for Israel."
"We will not stop standing by the side of the Israelis until they have won," he vowed, adding "There will be a ceasefire only when Hamas ceases to be a threat to Israel."
Former vice president Mike Pence surprised the gathering Saturday when he announced he was dropping out of the 2024 presidential race, becoming the first major candidate to suspend his campaign.
FILE - TOPSHOT - Former US vice president and Republican presidential candidate Mike Pence, with his wife Karen , at the RJC Annual Leadership Summmit after announcing he is dropping out of the race on October 28, 2023 in Las Vegas, Nevada.
"It's become clear to me: this is not my time," he said. "After much prayer and consideration, I have decided to suspend my campaign for president."
Support for Israel is a huge issue for both political parties in the United States, and a rare instance of foreign policy that matters at the ballot box, thanks in part to the large number of Jewish voters.
It is also a significant issue for evangelical Christians for whom the existence of a Jewish state is a key precondition for the hoped-for "second coming" of Jesus Christ.