BEIRUT—The main Lebanese factions in the political dispute that brought down the government here Wednesday are looking to Syrian President Bashar Assad for support—underscoring Syria's resurgence as a major player in the region.

Lebanon's Parliament is set to pick a prime minister on Tuesday after two days of debate in the legislature—but the country's political future could be decided in Damascus this weekend.

Prime Minister Saad Hariri was ousted from the post by opposition ministers led by Hezbollah, the militant Shiite political party, on Wednesday, over Mr. Hariri's refusal to reject a United Nations-backed tribunal investigating the assassination of his father, a former prime minister.

 A U.S. report leaked by WikiLeaks suggested Hezbollah believed Syria was responsible for his assassination in 2008.

The tribunal is expected in the coming weeks to indict members of Hezbollah for roles in the assassination. Damascus and Hezbollah, which also has Iran's backing, have sought to stall the indictments.

Lebanon's main daily newspaper, An Nahar, reported that Hezbollah ministers withdrew from the cabinet following a call by Mr. Assad for their resignations. The collapse created the worst political crisis Lebanon has seen in over two years.

Independent parliamentarian Walid Jumblatt, whose Druze bloc has enough seats to deliver a majority to whichever candidate he chooses, was set to meet with Mr. Assad in Damascus on Saturday.

Supporters of Mr. Hariri said they were hoping Mr. Jumblatt would convince the Syrian leader to endorse the prime minister's return to office. Mr. Hariri arrived in Lebanon on Friday after meetings seeking support in Washington, France and Turkey.

By Lebanese law, the prime minister must be appointed from the Sunni sect. Hezbollah has said it would block Mr. Hariri's reinstatement, and threatened to force new legislative elections, extending the political uncertainty, if he is chosen.

The political crisis positions Mr. Assad between Iran, which with Syria has long backed Hezbollah, and the U.S. and Saudi Arabia, which back Mr. Hariri.

The situation is a test of Washington's diplomatic outreach to Syria, an effort by the Obama administration to sway Damascus away from Iran's influence.

Syria's relations with the U.S. soured after the 2005 assassination of Lebanese Prime Minister Rafik Hariri. Washington recalled its ambassador to Syria after massive demonstrations in Beirut blaming Syria for the incident, a car bombing that killed 22 people.

Those protests led to the end of Syria's 30-year military presence in Lebanon.

Syria has since shown it is most likely to stand by Iran and Hezbollah. "Syria is keeping all its cards on the table but ultimately it will chose what is in its best interest—and its relations with Iran and Hezbollah are far more important than the United States," said Sami Baroudi, professor of political science at Lebanese American University.

Hezbollah and Syria have both denied charges that they played a role in the assassination.

They both stand to suffer a blow to their reputations among their constituents and Arab public opinion, if they are declared complicit in Mr. Hariri's assassination, because he was among the most popular Sunni politicians in the region.

Hezollah and Syria have tried to soften the impact of the allegations by discrediting the court—saying it was influenced by Israel and the U.S. Iran's Supreme Leader Ayatollah Ali Khamenei, spiritual leader of Hezbollah, said the court's findings were null and void.

But the tribunal has had substantial Western backing, including from the U.S., and while Saad Hariri has said he wouldn't support the prosecution of Hezbollah members, he has refused to reject the findings.

Syria and Saudi Arabia have tried for months to broker a deal between Hezbollah and Mr. Hariri, including efforts to convince Mr. Hariri to discredit the tribunal in the interest of stability.

When those talks reached a dead end, the Lebanese opposition ministers resigned, dissolving the government.

"The Saudi-Syrian efforts have played a positive role in maintaining truce in the country….A real opportunity to serve Lebanon that was lost," Saad Hariri said in a televised statement on Friday.