Republican Sen. Ben Sasse (Neb.) has not shied away from publicly chiding the GOP's presumptive presidential nominee Donald Trump, stepping up his attacks Wednesday after the business mogul's final primary opponent John Kasich bowed out of the race.

In an open letter published to Facebook, Sasse lashed out on Trump and his likely Democratic competitor Hillary Clinton, calling for an "adult" third party candidate to step into the ring.

"Neither political party works. They bicker like children about tiny things, and yet they can’t even identify the biggest issues we face," Sasse wrote. "They’re like a couple arguing about what color to paint the living room, and meanwhile, their house is on fire."

The Nebraska senator said he walked into a Walmart in his hometown Fremont one day after the Indiana primary elections, where Trump swept all 57 delegates up for grabs and was swarmed with angry comments from constituents about the nation's political state.

Recording four of his conversations, Sasse penned a 10-part breakdown of how both the Republican and Democratic parties have failed the nation, advocating for a one-term third party option who can work among varying political coalitions.

This open letter aims simply to ask: "WHY WOULD AMERICANS EVER SETTLE FOR A CHOICE LIKE THIS?" https://t.co/pIwmWunFSD — Ben Sasse (@BenSasse) May 5, 2016

At this point, I'd happily support a moderate. Would be more conservative than either of the two dishonest liberals now leading the Rs & Ds. — Ben Sasse (@BenSasse) May 5, 2016

Sasse said he ignored his phone all day Wednesday, leaving his voicemail inundated with messages from "party bosses" attempting to push the junior senator toward backing the GOP ticket because "‘although Trump is terrible' we ‘have to' support him ‘because the only choice is Trump or Hillary.'"

But Sasse didn't bite, responding instead that "there are dumpster fires in my town more popular than these two ‘leaders.'"

Trump's win in Indiana ended the possibility of a contested GOP convention in Cleveland, leaving his Republican competitors Sen. Ted Cruz of Texas and Gov. John Kasich of Ohio with no path forward.

Republican National Committee Chair Reince Priebus called on those in his party Tuesday to unify behind Trump to stop Clinton's momentum.

.@realDonaldTrump will be presumptive @GOP nominee, we all need to unite and focus on defeating @HillaryClinton #NeverClinton — Reince Priebus (@Reince) May 4, 2016

The call to arms did not stop Sasse, who has vowed since February that he would never back a Trump candidacy, from criticizing his party's complacency with the former reality star's nomination.