WASHINGTON — (CNN) Sen. Ben Sasse on Monday — a week before the Republican National Convention begins — lamented the lack of trustworthiness in the two likely presidential front-runners, and urged voters to follow their conscience, not pick between the lesser of two evils.

“To us, the act of voting is also a civic duty that tells people what we think America means, what we want to teach our kids about moral leadership, what face we want America to present to the world, and what sort of candidates we want more of in coming years,” he wrote in an essay posted on Medium, referring to himself as a “conscience voter.”

Sasse, a first-term Nebraska Republican, caught the attention of the GOP with a late-night, anti-Donald Trump tweet storm in late February, and some party elders had hoped he would mount his own third-party presidential bid. The senator demurred, but kept his voice in the discussion of how conservative voters should move forward, even as Trump tries to pull the party together.

Sasse’s essay is just another reminder that even as conservatives are set to gather in Cleveland, not all are rallying around the presumptive nominee.

At a minimum, Sasse said, one must trust that a new president will adhere to the oath of the office — to “preserve, protect and defend the Constitution.”

“Today, I do not have this confidence about either of the current front-runners. I think one of them does not even know what the Constitution is about, and the other doesn’t care,” he wrote in his 1,600 word essay that was a scathing indictment of both Trump and Hillary Clinton.

“By the way, this is a good time to say that if you really think one of the two presidential frontrunners is genuinely trustworthy, then fine, you should vote and sleep soundly,” he said. “Sadly, I do not regard either of them as worthy of our trust.”

Sasse said while he understands the “lesser of two evils” argument, he wants to explain why it doesn’t work for him.

“To us, voting is not merely about 1/130-millionth of deciding who should preside over 1/3 of the federal government from 2017 to 2021,” he said.

The-CNN-Wire ™ & © 2016 Cable News Network, Inc., a Time Warner Company. All rights reserved. (PHOTO: Ben Sasse campaign office/CNN)