(CNN) You know Election Day is closing in when the kids are trick-or-treating. And with the costumes out, the picture for the Senate is clearer than ever.
It is pretty clear from the polling that control of the Senate will likely come down to four races: Arizona, Georgia, Nevada and Pennsylvania.
The math is simple. Democrats need to win three of these four races to maintain control of the Senate. For Republicans, it's a slightly easier climb as they need to win only two of these four races.
But despite the ease of the equation, solving it is anything but easy. All of these races are well within the margin of error. Moreover, the states aren't all that similar in demographics, which means that it's plausible that any late movement or polling error could affect the states in different ways. Each state has unique issues affecting them, too.
Arizona is the easiest race to understand. Democrats have won the last two Senate races in the state, after not having won one since 1988. They're powered by increasingly strong performances in the Phoenix suburbs among White college-educated voters and a reliable Hispanic base. (They're also helped by one of the largest Native American populations in the country.)
Democratic Sen. Mark Kelly hasn't trailed in any poll released publicly. His advantage has slimmed in some recent surveys, though many of those are from outfits that don't meet CNN's standards for publications.
On average, Kelly has been up by about 3 points over Republican Blake Masters. A New York Times/Siena College poll published Monday gave Kelly a 6-point lead over Masters.
Masters' problem is fairly simple: His net favorability (favorable - unfavorable) rating is underwater. Unpopular Republican candidates is an issue that has plagued Republicans across the board. Meanwhile, Kelly's net favorability (and approval rating) has been positive.
This has allowed Kelly to overcome President Joe Biden's own unpopularity in the state.
Kelly's consistent lead may be why a number of analysts (though not this one) have viewed the Senate as coming down to the other three states.
The best one for Republicans and arguably the most straightforward one is Nevada. The Times poll there out Monday and the average has the race tied between Democratic Sen. Catherine Cortez Masto and Republican Adam Laxalt.
Nevada seemed to be a state trending toward Democrats 10 years ago, after Barack Obama won it with ease in two consecutive elections. Republicans have lost the last two presidential elections in the state by decreasing margins, including a 2.4-point loss in 2020.
Republicans have been helped by movement toward them among Hispanics, as well as a large base of White voters without a college degree. The state's economic base of tourism was hurt during the Covid-19 pandemic, when national Democrats were far more likely to push for Covid precautions.
Cortez Masto, unlike Kelly, has not carved out a base of popularity, according to the polls.
The final two states to the Senate math are the hardest to figure out. Georgia and Pennsylvania couldn't be more different in terms of their demographic math, even if they both voted for Biden by slim margins in 2020.
Pennsylvania is a Great Lake swing state in which Democrats must win a healthy share of White voters without a college degree. That's a group that has been running away from Democrats, which is why Hillary Clinton in 2016 became the first Democratic presidential candidate to lose the state since Michael Dukakis in 1988.
If border issues play an outsized role in a state like Arizona and a recovering gaming industry are pivotal in Nevada, the big non-inflation story in Pennsylvania is crime. Philadelphia, the most populated city in the state, has seen a jump in its crime rate over the last few years.
Republican Mehmet Oz has used the crime issue to close what was once a large advantage for Democrat John Fetterman in the Senate race.
Fetterman, though, has seemed to persevere, despite a stroke that left him off the trail for a period of time. He continues to nurse a small lead in the area of 2 to 3 points. The Times had Fetterman up 6 points, though much of that polling was taken before a debate last week that many viewed as a weak one for him.
Additionally, Republicans have tended to outperform their final polling the last few cycles.
Oz, for his part, has had a negative net favorability rating throughout the campaign, as he's had to fight off charges of being a carpetbagger.
While Pennsylvania (and Arizona and Nevada) may take a while to project, we should know who wins them by the end of November. That may not be the case in Georgia.
Georgia is unique amongst the four in that the candidate with the most votes needs a majority to win. Otherwise, there will be a runoff in December.
At this point, a runoff seems quite plausible. Democratic Sen. Raphael Warnock and Republican Herschel Walker are in a tight race. Neither one of them is anywhere close to 50% in the average of polls, with Libertarian Chase Oliver pulling around 3% of the vote.
The potential for a runoff isn't the only thing that makes Georgia unique. The Peach State has, by far, the largest Black population of any of these pivotal races. Democrats have made a comeback in this deep Southern state because of a growing Black population, and the movement to Democrats among White college educated voters in the Atlanta area.
Georgia, though, is the most Republican of any of the aforementioned states. Biden won it by the smallest margin of any state he won in 2020. In January 2021 runoffs, Warnock along with Sen. Jon Ossoff became the first two Democrats to win Senate elections in Georgia in a generation.
Warnock's 2021 election was a special one to replace Sen. Johnny Isakson, which is why he's up for reelection so soon.
Ultimately, Georgia may come down to the same thing that is occurring in most swing states this year. A Republican candidate in Walker who sports a net negative favorability rating with the backdrop of a deeply unpopular President.
Whichever matters most to the rare swing voter will probably decide the winner in Georgia and who wins control of the Senate.