The Senate battle over Ukraine aid has become a legacy-defining issue for Minority Leader Mitch McConnell as the Republican is facing questions over his grip on power.

The Kentucky lawmaker, a dominant force in Republican politics for two decades, insists that assistance to Kyiv should be part of a broad foreign-aid package, which proponents see as boosting its chances for passage.

That approach, while also favored by the White House, has run into resistance from some GOP colleagues who say Ukraine should be considered in its own bill—or rejected outright—testing McConnell’s ability to rally enough GOP votes to pass the measure through the Senate.

“This rises above your typical domestic political dispute,” said McConnell in defending his stance on Ukraine, in its second year of fighting invading Russian forces. “I think that the country’s future, and the Western world’s future, depends upon winning this. It’s going to take a long time to determine how that turns out. But I think this is the biggest issue in the world right now,” he said in a recent interview.