And by half, we mean one. But the fact we're writing about what looks like one, very-likely missed opportunity for Senate Republicans to unseat a Senate Democrat underscores just how tight their margin of error is to keep control of the Senate this year.

Republicans can't afford any mistakes, and Tuesday's GOP primary in Colorado looks like it qualifies. In that swing state, Sen. Michael Bennet is one of Senate Democrats' most vulnerable incumbents -- or at least he was supposed to be. Republicans stumbled to find a credible challenger, and a messy, chaotic primary ended Tuesday with the one candidate Senate Republican operatives really, really didn't want to win: El Paso County Commissioner Darryl Glenn.

Glenn is an Air Force veteran and self-described "unapologetic Christian constitutional conservative." He's a Donald Trump supporter (he has called Trump a "patriot") and a hard-line conservative who has said he wouldn't moderate his views in a general election nor would he work with Democrats if voters sent him to Washington.

Republican Senate candidate Darryl Glenn talks about his experience growing up black in Colorado in this campaign video posted on Facebook. (Darryl Glenn)

That leaves national Republicans with a bad taste in their mouths. The last time they nominated a very conservative candidate to challenge Bennet in this swing state, they lost a winnable race in 2010. And it means Glenn's Senate campaign will probably be almost entirely funded by tea party groups that made him a factor in this primary in the first place. A Republican political aide said Glenn probably can't expect help from them in the general election.

In other words, Senate Republicans are basically writing off Colorado. This is yet another break for Senate Democrats, who need to net four to five seats to wrest control of the Senate from Republicans. Even before this result, the map was falling in a way that was giving them the opportunity to do just that. Polls showed key races in states like Pennsylvania and Ohio close. Republicans hold 8 of our top 10 races most likely to flip parties. A key Republican super PAC just reserved $40 million in air time for key Senate races; Colorado wasn't on the list. (Nevada, the other GOP pickup opportunity thanks to retiring Senate Minority Leader Harry Reid, made the list.)

The irony is that for most of Colorado's Republican Senate race, Glenn was not considered a threat to the four other better-funded outsiders and establishment picks in the race. Sure, he was the conservatives' darling after a rousing speech in Colorado Republicans' party convention in April -- a speech that shocked and wowed many who heard it and propelled him to a coveted spot on the ballot, thanks to the state party's endorsement.

"All lives matter," he cried at the speech. (At the time, Glenn was one of three black candidates in the primary.)

But in the first three months of 2016, he raised a grand total of $3,811. His election night party was scheduled for a barbecue restaurant.

In the waning days of the primary, Glenn moved that party to a ballroom. He got a boost from stars in the conservative world. In May, the Senate Conservatives Fund launched TV ads for him. Sarah Palin endorsed him. Sen. Ted Cruz (R-Tex.) traveled to Colorado and held an event with him. As polls opened Tuesday, the race was still wide open, but most observers gave a slight edge to Glenn.

And he won. AP called the race for him, with Glenn winning 37 percent of the vote with 71 percent of precincts reporting -- a double-digit win.

Glenn turned a shoestring primary campaign into a victory. The problem is that he'll have to repeat that all over again to have a chance in the general election. It's a long shot -- so much so that his own party doesn't really see a path to victory.

In the overarching race for control of the Senate, Senate Republicans just took a big hit.