Two wars, each with high stakes and painful costs, are together posing a critical test of President Joe Biden’s core foreign policy vision: a vigorous reassertion of America’s post-World War II role as global leader.

That’s because the political passions stirred by the wars in Ukraine and Gaza are threatening the support he will need to put that vision into practice.

Mr. Biden sees the catalysts for both conflicts – Russian President Vladimir Putin’s invasion of Ukraine, and Hamas’ Oct. 7 rampage through southern Israel – as a single challenge. “Hamas and Putin,” he declared in a rare Oval Office address on Oct. 19, “want to completely annihilate a neighboring democracy.”

But key political constituencies, at home and abroad, are not convinced about that parallel.

At home, bipartisan support for helping Israel remains strong. But with regard to Ukraine, it is fraying. A growing number of Republicans in the House of Representatives, echoing former President Donald Trump, question continued U.S. support for Kyiv. The newly elected House speaker, Mike Johnson, this week proposed a funding package limited to supporting Israel.

Mr. Biden may yet muster sufficient Republican support for Ukraine.

But abroad, the controversial issue is U.S. support for Israel. And that support is straining coalitions that the president has been counting on to reassert American leadership in the face of the rival ambitions of Russia, China, and Iran.

The first sign of this has come in the response of many countries in the Global South to Israel’s all-out bombing campaign against Hamas in Gaza, which has flattened large areas and killed nearly 9,000 people, according to figures from local authorities.

Last year, Washington was able to woo the overwhelming majority of those countries into opposing, or at least staying neutral toward, Russia’s Ukraine invasion. But now, most of them are defying the United States and demanding a cease-fire that Israel says would give Hamas fighters an advantage.

In March 2022, a mere handful of countries sided with Moscow against a United Nations General Assembly resolution deploring Russia’s invasion, while 141 nations stood with the U.S.

Last week, by 121 to 14, the same body voted in favor of a “sustained humanitarian truce” in Gaza, rejecting Washington’s insistence on an explicit condemnation of Hamas’ surprise attack and hostage-taking. The U.S. stood isolated, even from some of its European allies.

World attention is showing signs of shifting away from last month’s attack on Israel to focus more on the humanitarian crisis and civilian casualties in Gaza.

That has fed growing grassroots protests across the Arab and Muslim world and beyond. Bolivia this week severed diplomatic ties with Israel, while Chile and Colombia recalled their ambassadors.

Of most immediate concern in Washington will be its Arab allies – key to Mr. Biden’s hopes, dashed for now by the Israel-Hamas war, of securing a peace deal between Israel and Saudi Arabia as part of a regional bulwark against Iran.

These Arab states still see Iran as a shared threat, and last month’s brutal attack by Iran’s proxy, Hamas, has reinforced that view.

But in public, they have increasingly focused on the need for a humanitarian pause in the Gaza fighting, so as to protect civilians and get aid into the area. In the longer term, they are insisting – after years of Israeli government disinterest and international diplomatic neglect – on a return to serious efforts to secure a two-state peace deal between Israel and the Palestinian territories.

On Wednesday, Jordan – a longtime U.S. ally, formally at peace with Israel since 1994 – recalled its ambassador in protest over the “Israeli war on Gaza” and the “unprecedented humanitarian disaster” there.

The Biden administration is not about to weaken its support for what it sees as Israel’s right to defend itself in the wake of the killing and abduction of hundreds of its civilians. But it is clearly aware of the diplomatic price it may pay for that support.

In an interview with Foreign Policy magazine this week, Ehud Barak, the former Israeli prime minister and defense minister, predicted that “within a week or two we will probably lose the support of public opinion in many parts of the free world, and within another two or three weeks we might lose support of many of the governments in the free world. I think that America will still be with us, but it will be more and more complicated for them to stay behind us.”

To reduce such complications, Washington has been redoubling efforts this week to get large-scale aid deliveries to Palestinian civilians, and to establish a credible “safe zone” in the southern part of Gaza, so far without success.

And while sharing Israel’s view that a full cease-fire would leave Hamas strengthened politically, Mr. Biden this week called for a “pause” that would allow the release of hostages Hamas holds. U.S. Secretary of State Antony Blinken, arriving in Israel on Friday, was expected to push the government to arrange more such pauses for humanitarian purposes.

“American values are what make us a partner other nations want to work with,” Mr. Biden said in last month’s White House address. “American leadership is what holds the world together.”

If the U.S. president can use that strength to ensure that humanitarian relief reaches the residents of Gaza, he will have greatly reinforced his bid to reassert his country’s global moral and political leadership.