Sen. Ben Sasse (R-Neb.), a freshman in office barely one year, took to social media Sunday night to declare that he would not support Donald Trump if he sews up the Republican presidential nomination and formally called for an independent challenge from the right as a conservative option to the real estate billionaire.

“I sincerely hope we select one of the other GOP candidates, but if Donald Trump ends up as the GOP nominee, conservatives will need to find a third option,” Sasse wrote in a roughly 1,600-word missive posted on Facebook, much of which he also threw into a series of tweets to his followers.

He flat-out stated that “I cannot support Donald Trump” and pledged to support “neither of them” if the two nominees are Trump and Hillary Clinton for the Democrats.

Other conservative commentators have made similar declarations in recent weeks, but Sasse became the highest profile elected Republican official to declare that he would not support Trump. His declaration came about 36 hours before the pivotal 11-state ballot of Super Tuesday, which includes a cluster of primaries in conservative southern states.

Having won three of the first four states, Trump is expected to run up large margins in most of the states Tuesday, and many Republicans are beginning to think about what life would be like with the mogul as their party’s presidential nominee.

Sasse comes from the anti-establishment wing of the party, prevailing in a 2014 primary over more experienced candidates. Sasse, 44, has not endorsed any of Trump’s rivals but has campaigned on behalf of both Sens. Marco Rubio (Fla.) and Ted Cruz (Tex.).

“Here’s where I’m at: If Donald Trump becomes the Republican nominee, my expectation is that I will look for some third candidate — a conservative option, a Constitutionalist,” Sasse wrote. “I do not claim to speak for a movement, but I suspect I am far from alone.”

Such a conservative challenge via an independent campaign would almost assuredly split the Republican vote and ensure the Democratic nominee a victory.

Here is the entire letter Sasse shared on Facebook: