Less than four months after the painful losses, Democratic officials have begun charting a path back to Senate control that runs through more than half a dozen blue and purple states where the presidential campaign is expected to boost Democratic turnout. But even in that favorable terrain, the party faithful fear they could fall short if marquee challengers don’t step forward, since their talent pool is shallow and they are trying to unseat a well-prepared group of Republican senators.

“The bench is short, but the aces are strong,” said Democratic donor John Morgan. He summed up the all-or-nothing outlook with another baseball analogy: “All you need is a right-hander that throws 99 mph, and you don’t need a bench.”

Needing to gain four seats — five if a Republican wins the White House — to reclaim the majority, Democrats are under intense pressure to enlist top contenders. Since House Republicans hold their widest majority in decades and are early favorites to stay in power, the fight for the Senate stands to determine whether the next president will face a split Congress or one controlled completely by the GOP.