GOP senator unloads on Donald Trump in fiery Facebook manifesto

While many Republicans are slowly beginning to coalesce around presumptive Republican presidential nominee Donald Trump, at least one prominent conservative figure has refused to budge: Sen. Ben Sasse of Nebraska.

In a fiery open letter published late Wednesday night on his Facebook page, the junior senator reiterated his opposition to both Trump and Democratic presidential frontrunner Hillary Clinton, calling for a third-party challenger.

Detailing a meeting with people at his hometown Walmart, Sasse said there was clearly an "appetite" for a third-party candidate.

"I've ignored my phone most of today, but the voicemail is overflowing with party bosses and politicos telling me that 'although Trump is terrible,' we 'have to' support him, 'because the only choice is Trump or Hillary,'" Sasse wrote.

"This open letter aims simply to ask 'WHY is that the only choice?'" he continued.

The senator went after both political parties. He claimed both Republicans and Democrats did not sense the anxiety among constituents about both a number of policy issues and a general-election matchup between Clinton and Trump. 

"They're like a couple arguing about what color to paint the living room, and meanwhile, their house is on fire," Sasse wrote of both major political parties. "They resort to character attacks as step one because they think voters are too dumb for a real debate."

In the letter, Sasse listed a number of qualities that he would like to see in a third-party challenger, including a strong national-security background and a commitment to slashing entitlement spending.  

The senator also suggested an outside challenger could gain legitimate traction because of the two party frontrunners' record-high unpopularity.

"There are dumpster fires in my town more popular than these two 'leaders,'" Sasse wrote.

Following Trump's win in Indiana and Sen. Ted Cruz's decision to exit the race on Tuesday, Sasse took to Twitter to reiterate his opposition to Trump.  

Sasse is part of a vocal group of conservative thinkers and lawmakers who have refused to back the presumptive Republican presidential nominee over his perceived lack of conservative bona fides.

On Tuesday, a former top aide to Sen. John McCain's presidential campaign tweeted out his reluctant support for Clinton. Frequent Trump critic Sen. Lindsey Graham of South Carolina indicated at a conference the same day that the former secretary of state was likely to win the nomination. 

It appears unlikely at the moment that Sasse's desire for an independent candidate will be met. A third-party challenger would face major institutional obstacles.

Beyond the opposition of both political parties, a third-party challenger would also need to collect thousands of signatures in each state within a short amount of time to get on the ballot. It would amount to daunting task for a candidate without an existing organizational campaign structure.