Sasse, a Nebraska Republican, has emerged as a central figure in the movement. He wrote a Facebook post early Thursday
arguing for an alternative candidate to Trump and Clinton, which had garnered more than 6,400 likes by late morning.
"Why shouldn't America draft an honest leader who will focus on 70% solutions for the next four years? You know ... an adult?" Sasse wrote.
Trump's effective securing of the nomination has scattered Republicans, with some including Senate Majority Leader Mitch McConnell and party Chairman Reince Priebus urging their colleagues to unite behind him. But the disparate anti-Trump forces remain split -- with some saying they will vote for Hillary Clinton and others urging an alternative to Trump and Clinton, even if it means losing the White House.
"At the very, very least we're all going to unite to help the down-ballot candidates who are going to have an extreme amount of trouble right now," said Deborah DeMoss Fonseca, spokeswoman for Conservatives Against Trump on Thursday. "I think there will be a lot of people who will sit out the presidential one and try to help whoever we can help, but I can tell you there is a growing number of people looking at getting someone else on the ballot."
A spokesman for Sasse, 44, said later that the senator would not seek the White House. But that doesn't mean he wouldn't have support.
"I hope Ben Sasse decides to pick up the mantle himself. I think it would be fantastic to have a young, charismatic, conservative, but very independent-minded and willing to work across the aisle senator on the stage with two elderly Americans: Donald Trump and Hillary Clinton," conservative leader Bill Kristol said Thursday on CNN's "New Day."
Kristol said he would face challenges, but could find a path to the White House.
"Someone like Sasse running would start off behind, there's a lot of hurdles to get over. It's doable though and if he could get in the debates in September and October I think he could actually win," Kristol said.
Ben Howe, a senior editor at the conservative site RedState.com, said that he would effectively be supporting Hillary Clinton because their efforts to block Trump are aligned.
"If he has the bully pulpit for four years, defining conservatism, I don't know how we recover from that," Howe said
. "So it's not that I want Hillary Clinton to be president. It's just that I can't sit by and watch somebody co-opt conservatism and destroy its credibility."
Erick Erickson, a leader of Conservatives Against Trump, told CNN Wednesday that leaders looking a for a third option for the White House understand they have a tight window and must act fast.
"If the delegates ratify this madness in Cleveland, many of us will look elsewhere for a credible candidate to oppose both Trump and Clinton," Erickson said on CNN's "New Day."
"We will begin now laying the groundwork for an exit strategy from Donald Trump's Republican Party."
Wisconsin conservative radio host Charlie Sykes, who helped defeat Trump there last month, said on "New Day"
that Trump was "racist" and a "xenophobe" and "If you endorse Donald Trump you own all of that."
Conservatives Against Trump has multiple conference calls planned through the end of the week and has been debating every day over email. The group has smaller committees within it, focusing separately on delegate strategies at the convention, fundraising, reaching out to Republican leadership and overall campaign strategy, she said.
But no one has yet coalesced around one viable alternative candidate.
"We've never had a one-hour power session saying 'Who all can we get around?'" Fonseca said. "There are people who are saying 'Look, I was critical of Mitt Romney, but I would love to see Mitt Romney right now.' There are people now who didn't like Jeb Bush who are saying "Gee, I'd give anything to have Jeb back.' I'm like 'Nice time to say that.'"