Israel had created enormous political trouble for Biden. Then Iran attacked.

The attack on Israel came after weeks of Biden ratcheting up pressure on the Jewish state to scale back its Gaza military operations. That position found rare agreement from Donald Trump, who as the presumptive Republican presidential nominee has told Israel to complete its attacks there as quickly as possible.

But the politics surrounding the Middle East conflict were upended by the weekend’s events, which left some Republicans calling for the U.S. to retaliate militarily against Iran while the Biden administration urged only diplomatic responses.

Iran’s attack further complicated Biden’s political situation not just because of the possibility of a wider war, but because it might embolden Hamas to refuse a cease-fire deal with Israel. The fighting between Israel and Hamas that started in October has weakened Biden’s support with some key constituencies because he has continued to support Israel’s military assault despite growing numbers of civilian casualties.

“Peace and quiet benefit incumbent presidents," said Ari Fleischer, who served as press secretary for Republican President George W. Bush. “Turmoil, violence and a growing sense that international events are out of control hurt incumbents."

Democratic pollster Jeff Horwitt said Biden’s actions have reflected broad public sentiment.

“After two decades of failed wars, Americans want a president that shows strength through diplomacy, keeps America out of direct involvement, and our military out of harm’s way," he said. “That’s exactly what President Biden is doing in navigating the Middle East conflict and seeking to prevent a wider war."

Dan Gerstein, a Democratic strategist and former aide to Joe Lieberman when he was a senator from Connecticut, said Iran’s actions could buy Biden and Israel more time.

“Iran did what no other political actor could do—changing the narrative around Israel from bully to victim and rallying the sensible international center to Israel’s side," he said. “In doing so, they gave Biden a temporary gift and some breathing space to find a longer-term solution to the Gaza war."

Military aid is expected to dominate Capitol Hill this week after House Majority Leader Steve Scalise (R., La.) said his chamber would “consider legislation that supports our ally Israel and holds Iran and its terrorist proxies accountable."

House Speaker Mike Johnson (R., La.) said Sunday on Fox News that lawmakers would “try again this week" to pass an Israel aid bill, while also suggesting the House would look to resolve a longstanding fight over Ukraine aid. The Senate passed a $95 billion measure earlier this year that linked aid for Israel and Ukraine, but the bill stalled in the narrowly divided House.

Johnson, who met Friday with Trump in Florida, said he and the former president are “100% united on these big agenda items" and pointed to the idea of turning at least some of the Ukraine aid into a loan, while trying to seize Russian oligarchs’ assets to pay for Ukraine.

“These are ideas that I think can get consensus," he said. “We’ll put something together and send it to the Senate and get these obligations completed."

The U.S. response to the Middle East conflict will play out as Trump is set Monday to become the first former president to go on trial for criminal charges, with jury selection getting under way in a Manhattan courtroom. The trial on charges that he directed an illegal hush-money scheme to pay off a porn star is the first of four that he faces, with the three others related to attempts to overturn the 2020 election and the mishandling of classified documents.

Trump, who has denied any wrongdoing in the cases and repeatedly suggests they are politically motivated, kept his focus on the court proceedings in his social-media posts on Sunday.

During a Saturday evening rally in Pennsylvania, Trump said he would “revive American strength" and claimed the attack on Israel was because “great weakness" has been displayed by the Biden administration.

“It would not have happened if I were in office," he said, without offering any evidence.

Rallygoers repeatedly chanted “Genocide Joe" shortly after Trump addressed the attack on Israel, prompting the former president to respond, “They’re not wrong."

Some of Biden’s critics—many of them Democratic progressives—have used the slogan to describe him because of his support for Israel’s bombardment of Gaza and the soaring civilian death toll.

The attack by Iran on Israel and economic data released last week made clear that neither the Middle East nor inflation are cooling off as quickly as Biden’s re-election campaign would like.

Neither issue is completely within the Democratic incumbent’s control, but both could weigh heavily on the mood of voters if conditions don’t improve significantly by November.

While polls show the pocketbook issue of inflation is more central to Biden’s re-election challenges, continued tension in the Middle East is also problematic because it leaves Americans fearing global violence and uncertainty and exacerbates divisions within his own party over Israel policy and the suffering of those in Gaza.

Continued tensions in the region could also push oil prices higher. Gas is 50% more expensive than when Biden took office, after prices started to move higher again at the start of this year.

A CBS News/YouGov poll released Sunday that was conducted before Iran’s attacks showed just a third of American adults approve of Biden’s handling of the Israel-Hamas conflict, down 5 percentage points since February. Only a quarter supported the U.S. taking military action of its own against Iran, if that nation attacked Israel.

Catherine Lucey and Vivian Salama contributed to this article.

Write to John McCormick at mccormick.john@wsj.com