Presidential candidates set the turnout. That's the race voters care about the most by far, so everybody else who's running down-ballot essentially has to work within the structure of that year's presidential turnout numbers.
Some organizations can fiddle around with the margins here and there and get some voters out who might not have bothered voting, but down-ballot candidates mainly have to find a way to win with the hand they are dealt by the top of the ticket. So, if one presidential candidate crashes and burns, that's almost always the ballgame.
That's just something to keep in mind while watching top Republicans abandon their presidential nominee Donald Trump after that ghastly "hot mic" tape surfaced and he, in turn, lashed out at those turncoat Republicans and promised a nuclear attack on the Democratic nominee whom he has pledged to prosecute if elected.
Yes, there's still time for Trump to get his act together. The election is still a few weeks away – usually considered an eternity in politics. But, man, just look at this mess. Miracles do happen in politics, but miracles ain't plans.
It goes without saying that the impact of a possible implosion in Illinois will be felt the most in the suburbs, where more moderate voters were less receptive to Trump's candidacy to begin with. Rep. Sam Yingling, D-Grayslake, has been under constant, withering attack from the Republicans for months, so he could really use some help from the top.
The Democrats recently sent more than $200,000 to Cynthia Borbas' DuPage County race against Rep. Christine Winger, R-Wood Dale. That's about half of all the money they've given her since July. President Barack Obama lost the district by two points in 2012, so it's worth watching if this weirdness at the top continues.
The Republicans maintain that their expensive and meticulously concocted plan, centered around tying Democratic candidates directly to the horribly unpopular House Speaker Michael Madigan, will get people to the polls and convince them to support Republican legislative candidates even if some of them vote for Hillary Clinton.
But the Republicans have also said for months that their plan depends on Trump not completely falling apart. He just has to run a "normal" race, which obviously isn't happening.
It’s hard to see how this will work out well for the Republicans in the suburbs, where moderate women voters are undoubtedly appalled at what’s going on with Donald Trump.
But it’s a bit trickier in downstate districts, where Trump has been the most popular here.
For example, just a couple of short weeks ago, Mike Mathis' Democratic supporters were proudly pointing to the number of homes that had their guy's yard signs planted next to Trump's signs. Mathis is up against appointed Rep. Avery Bourne, R-Raymond, is a district just south of Springfield and has run a credible race.
The Mathis campaign's boast about attracting Trump supporters has also happened in most every other contested downstate district, as what remains of the Democratic blue collar white male vote surged away from Clinton to Trump. Simply put, the Democrats recognized early on that they needed those votes to win.
In Mathis' case, President Obama lost the 95th House District by 13 points four years ago, so the only way he can defeat Bourne is with support from people who normally lean Republican.
Mathis was asked about who he was supporting for president at a candidates’ forum last week. After much hemming and hawing and complaining about how bad the choices were, he finally said, reluctantly, that he’d be voting for Hillary Clinton.
Trump has become so toxic that maybe Mathis’ position won't hurt him too badly. Then again, lots and lots of Trump supporters believe Clinton is the Devil incarnate (I'm not kidding, do a Google search). They may not like what their guy is doing, but that won't push them toward her.
Trump polled a mere 29 percent in a statewide Democratic poll taken the middle of last week and careened downward to an unheard-of 27 percent in a Republican tracking poll earlier last week. He bounced back to above 30 in that GOP tracker, but ... oh, man that’s still bad.
To be clear, a Trump implosion will undoubtedly help Democrats. The fewer people who lean Republican who actually vote on election day or switch parties means Democratic voters will make up a greater share of the November 8th pie.
But after positioning themselves as Trump-friendly for months, Downstate Democrats now have to navigate a minefield for the next few weeks.
• Rich Miller also publishes Capitol Fax, a daily political newsletter, and CapitolFax.com.