Democrats have not gotten much good news this primary season as the GOP has mostly avoided bitter primary fights pitting the tea party against the establishment.
Amazingly, the bitter fight and the likelihood that McDaniel will prevail (his supporters are more likely to turn out in a runoff) puts the deep-red state of Mississippi in play in the general election, which will thrill Democrats who face the real prospect of dropping six seats and with it control of the Senate.
Democrats nominated centrist former Rep. Travis Childers and say their internal polling indicates he would start a campaign against McDaniel in a statistical dead heat. That's pretty remarkable (and perhaps a bit hard to believe) for a state as conservative as Mississippi. And even if Childers ultimately loses, a McDaniel race could force the GOP to spend time and money on a seat it once considered safe, possibly draining resources from other competitive contests.
And that leaves the Republican Party and its establishment allies with a very difficult conundrum. Do they heavily support Cochran in the runoff, as they did in the primary, and risk damaging McDaniel, who could wind up their nominee?
That seems to be the approach so far. Rob Engstrom, political director of the U.S. Chamber of Commerce, tweeted on Wednesday morning that this organization "will stand by Senator Cochran."
Read More Crucial Mississippi GOP Senate race heads to runoff
Engstrom then went further, sharply criticizing McDaniel in a subsequent tweet: "Chris McDaniel is a personal injury lawyer. Personal injury lawyers are not conservative."
Assuming Cochran stays in the race—some have speculated he might step aside—that kind of language will be commonplace on the air and on the ground in Mississippi for the next three weeks.
Should McDaniel ultimately prevail, Childers and the Democrats will face an opponent already shredded by prominent Republicans. Hard to see how the GOP establishment, once it knocks McDaniel down, will be able to pick him back up again in the fall.
And conservative activists are clearly unwilling to stand down and allow Cochran to have the nomination.
The conservative Club for Growth issued a sharply worded statement on Wednesday calling for Cochran to get out of the race. "Senator Cochran has served honorably, but the rationale for his candidacy ended yesterday," said the group's president, Chris Chocola. "He should do the honorable thing and decline to contest the runoff. Should he choose to persist, the Club for Growth PAC and conservatives throughout Mississippi will vigorously pursue this race to its conclusion, and we will look forward to the election of Senator Chris McDaniel."
It's hard to imagine the Mississippi contest getting uglier—it has already involved McDaniel supporters getting arrested for breaking into a nursing home to photograph Cochran's 72-year-old wife, who suffers from dementia—but that's exactly what is likely to happen over the next three weeks.
The only thing that could stop that would be Cochran, who initially suggested he might not seek another term, deciding not to contest the run-off. The 76-year-old senator did not give a speech Tuesday night. But his campaign tweeted: "New campaign starts tomorrow. Three weeks to victory."
But while the Mississippi dogfight bodes well for Democrats, the results Tuesday night were not all favorable for the party.
In Iowa, Jodi Ernst, who rose to fame for an ad in which she spoke of her prowess at castrating hogs, trounced her primary foes to emerge as the GOP nominee for the opening seat being vacated by Democratic Sen. Tom Harkin. Ernst picked up 56 percent of the vote and will now have the momentum against Democratic Rep. Bruce Braley in the general.
Republicans also dodged a bullet in California, where former Treasury official (and TARP administrator) Neel Kashkari emerged with the GOP nomination for governor and the right to lose badly to Democrat Jerry Brown in the fall. Had Kashkari lost, the GOP would have had to deal with state Assemblyman Tim Donnelly as their nominee.
Donnelly calls himself a "patriot, not a politician" and is given to driving around the state in an RV and firing guns around reporters. He once referred to immigrants rights groups as an "insurgency" that need to be "rooted out." That's just the kind of nominee the GOP is hoping to avoid on statewide tickets in 2014.
But the big headline of the night was certainly in Mississippi, where Democrats are hoping to pop some popcorn and watch three weeks of GOP on GOP crime.
—By Ben White. White is Politico's chief economic correspondent and a CNBC contributor. He also authors the daily tip sheet Politico Morning Money [politico.com/morningmoney]. Follow him on Twitter