Democratic presidential frontrunner Hillary Clinton has attacked Republican contender Donald Trump for taking a neutral stance on Israeli-Palestinian peace efforts, in a preview of a possible election battle between them.
As Mr Trump visited Washington on Monday, Ms Clinton told the American Israel Public Affairs Committee (AIPAC) conference that Mr Trump's neutral stance would be dangerous for Israel, a stalwart US ally in the Middle East.
"America can't ever be neutral when it comes to Israel's security and survival," Ms Clinton told the pro-Israel lobbying group, without mentioning Mr Trump by name. "Anyone who doesn't understand that has no business being our president."
Mr Trump, the Republican frontrunner, was to address the AIPAC conference later in the day, along with his Republican rivals, US Senator Ted Cruz of Texas and Ohio Governor John Kasich.
Ms Clinton's Democratic challenger, US Senator Bernie Sanders, was not appearing at the event.
Mr Trump has drawn fire for his position on Middle East peace negotiations. The New York billionaire has described himself as extremely pro-Israel but has said he would take a "neutral" stance in trying to negotiate an elusive peace settlement between Israel and the Palestinians.
Mr Trump's critics have said a neutral position could harm long-standing US support for Israel. Ms Clinton said she would make it a priority if elected to preserve the US-Israeli relationship, ensuring Israel had a qualitative military edge.
"We need steady hands, not a president who says he's neutral on Monday, pro-Israel on Tuesday, and who-knows-what on Wednesday because everything's negotiable," she said.
Ms Clinton, a former Secretary of State, also took aim at Mr Trump's vow that, if elected, he would deport illegal immigrants and bar Muslims temporarily from entering the US.
She noted an incident during the 1930s, when the US refused entry to a shipload of Jews trying to escape Nazi tyranny.
"We've had dark chapters in our history before," Ms Clinton said. "We remember the nearly 1000 Jews aboard the St. Louis who were refused entry in 1939 and sent back to Europe. But America should be better than this. And I believe it is our responsibility to say so."
Rick Jacobs, president of the Union for Reform Judaism representing roughly 1.5 million American Jews, praised Ms Clinton for her command of the issues and said he hoped Mr Trump revealed specific policy goals as well as a coherent philosophy of the US role in the Middle East.
"I will be listening very carefully to what he says and what he doesn't say. Can he put forward a very clear set of commitments that will help us understand him?"
Mr Trump's rise has alarmed establishment Republicans who have tried in vain to stop him.
Their best hope of derailing his insurgent candidacy is to stretch the contest out and deny him the 1237 delegates needed to formally win the party's presidential nomination.
Such an outcome would mean that the nominee for the November 8 election would be decided at the party's convention in Cleveland from July 18-21.