But here's a new overriding wrinkle to keep in mind: The most important factor deciding whether the Republican Party maintains control of Congress is actually President Trump himself and his behavior. He can Twitterize any perceived opponent he wants. But at the end of Nov. 6, 2018, he must own the results and live with them for two long years.
History suggests - but does not guarantee - the election outcome will displease Trump. Here's why: Since the Civil War, the president's party has lost House seats in 36 of 39 midterms, on average more than 30 each time. That's six more than Nancy Pelosi needs to grab the Speaker's gavel back from Paul Ryan come 2019.
That's the bad news. The good news for Trump is two of those three loss exceptions have come recently - in 1998 when Clinton Democrats took back five House seats, and 2002 when Bush's GOP gained eight House and two Senate chairs.
Off-year elections are actually a fascinating element of American politics. With no parliamentary vote of confidence available to grade a president halfway through a term, the biennial ballot outcomes for all House seats and a third of the Senate have become political thermometers for a president's popular standing.
History shows by that time U.S. voters are usually grumpy about a White House occupant. In his first midterms in 2010, Barack Obama's Democrats got spanked, losing an historic 65 seats and the majority.
In Bill Clinton's first midterms, Republicans' 1994 "Contract with America" helped the GOP gain 54 House seats, eight Senate seats and 10 governorships, including some Texan named George W. Bush. The lower a president's job approval the worse the midterm losses are.
Trump's approval rating has bounced around recently from the mid-thirties to low forties, not strong by any measure. And responses show people would prefer less turmoil and outrage in public life.
Trump lavishes attention on this base, and it's shown sturdy and puzzling resistance to his numerous, at times outlandish controversies. I suspect that's because such behavior merely confirms for them that he is the D.C. disrupter they crave.
However, 40 percent approval also means nearly 60 percent disapproval, actually around 57 percent. That's a nearly 17 percent deficit.
Here's a wrinkle: Polls on voters' party preference for a congressional representative next year give the GOP only a seven-point deficit, likely indicating they see Trump's Republican connections as tenuous, an accurate assessment.
Another wrinkle: Trump's weak approval flies in the face of positive news. Stock markets regularly record historic highs. After eight years of anemic economic growth, the last two quarters came in at a strong three percent, even with hurricanes. As Trump promised in the campaign, ISIS is crumbling before the U.S.-led assault.
With Obama era regulations being eased or erased and the prospect of significant tax reforms, optimistic employers are expanding and hiring.
So, why isn't Trump's standing improving with a majority of Americans? The answer should be clear, even if it must penetrate a New York-sized ego. It's Trump himself, his self-destructive behavior, gratuitous fights with would-be allies and peremptory retorts that distract attention from his own important initiatives and hand opponents easy ammo for credible criticism.
Trump supporters say he's a counter-puncher, always hitting back for the last word. Even Hillary Clinton, no great master of human insight, noted how very easy Trump is to bait.
Always counter-punching may be fine in boxing. But in the eyes of many, not in the world's most powerful commander-in-chief. It instead reveals within a leader an unattractive, even disturbing, insecurity.
Trump thinks his supporters love it. Maybe. Or maybe they just endure it. Either way, that other 57 percent clearly dislikes it. To sell his agenda, Trump needs at least to dilute that opposition.
As unlikely as it would seem for a 71-year-old brash billionaire, Trump needs to change his public behavior. Hard to detect things that don't happen, but he actually has toned down carping over the Mueller probe. Perhaps credit his lawyers.
Policies aside, Americans basically want to like their president. Now, for his own good this president needs to apply that same self-discipline across the board to let a wider public peer through the fog of his anticipated arrogance and discover for themselves the smart, confident, surprisingly kind man he is.
Without that, we're witnessing a 21st century Shakespearean sequel in which the protagonist Donald J. Trump is the one who defeats Donald J. Trump.
McClatchy Washington Bureau