ROME — The survival of Premier Mario Draghi’s government hung in the balance Thursday before a crucial Senate vote on a relief bill for soaring energy costs, after a key ally vowed to boycott the measure.

While senators publicly debated the bill, which extends gasoline subsidies expiring within days, backdoor maneuvering by allies in Mr. Draghi’s 17-month-old pandemic unity government explored possible strategies to avoid risking the former European Central Bank chief stepping down.

On Wednesday night, the former premier, Giuseppe Conte, announced that his splintering, populist 5-Star Movement would boycott the vote.

If the government puts the bill to a confidence vote, as first hypothesized, and loses, Mr. Draghi would be forced to tender his resignation to the Italian president, who tapped him to replace Mr. Conte’s tottering premiership in February 2021.

While the government didn’t yet formally call for a confidence vote, his Minister for parliamentary relations, Federico D’Inca’, a 5-Star who opposed Mr. Conte’s decision, said Mr. Draghi would indeed tie his coalition’s survival to the relief bill.

The 5-Stars oppose a provision of the bill to allow Rome to operate a garbage incinerator on the outskirts of the chronically trash-choked Italian capital. Mr. Conte has refused allies’ cajoling to budge, even after Foreign Minister Luigi Di Maio recently broke from the Movement over the populists’ stance, forming his own party.

The vote on the bill, which reduces taxes on gasoline and diesel fuel, as well as extends utility bill relief, was being held in early afternoon.

Without the votes of the 5-Stars, the bill presumably could muster enough support to pass. But even without braving a confidence vote, Mr. Draghi would be tempted to resign anyway, since he has repeatedly made clear that the populists were among the coalition partners, ranging from left to right, that signed up to be part of his government.

President Sergio Mattarella could accept or reject any resignation by Mr. Draghi. The president could also ask Mr. Draghi to go before Parliament in the coming days to seek a formal vote on the government itself, to see if the ranks of squabbling allies would rally around him to keep the premier in office.

Parliament’s term expires in spring 2023. If Mr. Mattarella can’t come up with a solution so Mr. Draghi’s government can continue, he is expected to dissolve the legislature and call an early election, which could come as early as late September.

In the debate, several senators blasted Mr. Conte’s decision.

Being in a government “is not like picking up a menu and deciding, antipasto, no, gelato, yes,’” said Emma Bonino, who leads a tiny pro-Europe party.

Others noted that Mr. Draghi had increasingly become a pivotal figure in Europe, while Russia wages war against Ukraine, especially with the downfall of Prime Minister Johnson.

An ally of the center-right leader and former Italian premier, Silvio Berlusconi, argued in the Senate that a collapse of Mr. Draghi’s government could trigger “the destabilization of Europe.”

“You’d be doing a favor to Putin,” thundered Senator Antonio Saccone against the 5-Stars.

Mr. Draghi has governed with the support of virtually all of Italy’s main parties, with the exception of the fast-rising far-right Brothers of Italy party, which is demanding that Mr. Mattarella pull the plug on Parliament and give Italians their say at the ballot box.

Among Mr. Draghi’s achievements has been keeping Italy on track with reforms that the European Union has made a condition for the country to receive 200 billion dollars in pandemic recovery assistance.