So, how did we get here? If you start the clock right after the 2016 elections, you see a pretty clear narrative, one in which Republicans, who did not expect to win, repeatedly made decisions with high political costs. You see Democrats acting more decisively and coherently than they usually do, focusing for nearly two years on health care, even as shiner objects flew across their screens.

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November 29, 2016: President Trump picks Tom Price to lead HHS

The president plucked five Republicans out of House and Senate seats for roles in his administration, setting up special elections that lasted until December 2017 — all of them expensive, all of them close. The elevation of Price, a doctor who had been seen as a Republican point man on health-care restructuring, had the biggest aftershocks, starting with the secretary's ham-handed efforts to sell Affordable Care Act repeal plans, continuing with the eight-figure cash fire that was Georgia's 6th District special election, and ending with Price becoming the first member of the Cabinet to resign in disgrace. Oh, and that district is, once again, a toss-up, after years of Price winning it comfortably.

November 30, 2016: House Democrats reelect Nancy Pelosi as leader

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Weeks after losing the presidency, House Democrats held a close, bitter contest to decide their future, and they decided to stay the course. Just a third of them rejected Pelosi in favor of Rep. Tim Ryan (D-Ohio), a vote that had two consequences.

First, it changed how Pelosi herself approached a job she'd held for 14 years, leading her to assiduously promote younger Democrats to speak for the party on bills and in high-profile media. Second, it locked the Republican Party and its PACs into the idea that the midterm could be framed as a choice between Trump and Pelosi. Both decisions defined the parties for two years; with no popular figurehead, Democrats largely built their own brands in tough races.

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How surprised was the political establishment when this event became the biggest one-day protest in American history? Most of the Democrats running to lead the DNC were, at that moment, in Florida for a big donor meeting. The speedy liberal organizing that followed the 2016 election really became unmistakable at this protest, the first in a wave of them, which helped organize and train activists even as some of the causes petered out and as the Women's March organization itself was occasionally hit with controversy.

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This was the first in what became a wave of 28 farewell parties for House Republicans, and it was an especially rough one. In one moment, a seat that had been safe for the party so long as Ros-Lehtinen was their candidate became a first-tier Democratic pickup opportunity, something the Republican knew when she retired. And Ros-Lehtinen was the first of several Republicans who, on the way out, were happy to talk about how the Trump-era party was unrecognizable to them. After a September scare, Democrats believe they've saved the seat for their candidate, Donna Shalala, with one simple ad that focuses on Ros-Lehtinen's successor, Maria Elvira Salazar, tweeting “Bravo, Trump!”

May 4, 2017: House Republicans celebrate the passage of the American Health Care Act in the Rose Garden

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Every single aspect of last year's health-care battle has come to haunt Republicans, but none so much as the way the party reacted to its Pyrrhic victory in the House. After rescuing the bill with an amendment from Rep. Tom MacArthur (R-N.J.) and eking out a House vote, Republicans piled onto buses and headed to the White House for a celebration that put nearly all of them on camera, with the president, laughing and joking. Dozens of Democratic candidates have said that the moment spurred them to run; footage from the event has fed millions and millions of dollars in ads. MacArthur's own race is now a toss-up.

The firing of James B. Comey as FBI director was a watershed event, though not in the way Democrats first expected. It ended a fantasy that had bedeviled the party for months, of Republicans suddenly turning on Trump and demanding that he leave office; it made it clear that no event could make that happen. For electoral purposes, it gave Democrats an answer to any question about whether they would impeach President Trump. For all but a few Democrats (and almost none in tough races), the answer became: Wait until Mueller finishes. Republicans, with long memories of how the campaign to impeach President Bill Clinton temporarily backfired on them, kept expecting Democrats to overreach. The Mueller probe gave them something else to say.

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To Republicans, Ossoff is still a punchline, an ambitious candidate who wasted his party's money. But his race proved to Democrats that their base was unusually alert and ready to donate to causes, any causes, so long as they could be convinced that there was a chance to grab power back from Republicans. The president's party spent far too much time telling itself that the big cash hauls for Democratic House candidates would peter out. They never did.

August 15, 2017: Roy Moore makes it into the GOP primary runoff in Alabama

It's remarkable to think about the sequence of events that put Sen. Doug Jones (D-Ala.) into Jeff Sessions's old office. What was clear last summer was just how unsettled the Republican Party was and how the party was struggling to tame it. To shore up Luther Strange, the man appointed to replace Sessions, Republicans unleashed attack ads that pushed Rep. Mo Brooks (R-Ala.) out of contention; ads that focused on how, like many conservatives, he'd criticized Trump when he was seeking the GOP nomination.

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The theory, which proved disastrous, was that Strange could easily defeat Roy Moore, who sailed into the runoff and was considered by many national Republicans a clownish has-been. This was the first of several primary plans that fell apart; another that could resonate Tuesday was the effort in West Virginia that crushed Don Blankenship but opened a path for Patrick Morrissey to become a flawed challenger to Sen. Joe Manchin III (D-W.Va.), and for Leah Vukmir to become an unexpectedly weak challenger in Wisconsin, a state that had once looked so promising for Republicans.

This was the first of a half-dozen resignations or retirements in what would be seen as a #MeToo moment for Congress. Murphy's decision, which set up a special election that Democrats would go on to win, was the most impactful in electoral terms. It also set a kind of standard: Scandals that members might have been able to ride out before became impossible to survive, as everyone from John Conyers Jr. to Trent Franks to Al Franken would find out. Only one of those seats changed parties in a special election: Murphy's.

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The survival of New Jersey's senior senator created a naggingly close election for his party Tuesday. It also, surprisingly, set a kind of standard for campaign scandals this cycle: In the Trump era, some kinds of behavior was survivable. Republicans now expect Menendez to prevail and two indicted members, Rep. Duncan D. Hunter (R-Calif.) and Rep. Chris Collins (R-N.Y.), to win reelection.

The Pennsylvania Democrat was not the first challenger to renounce Pelosi, but he did so in a way that other candidates would emulate, with a big, showy announcement in local newspapers. That decision reduced the utility of the Republican campaign to portray Lamb as just another Democratic puppet; this is why dozens of Democrats, many in good positions to win tomorrow, would copy it.

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More than any other moment on this list, this reset the race for the House by turning Pennsylvania from a state heavily slanted toward the Republicans who'd drawn its 2011 map to an evenly balanced state where any party winning statewide would win a majority of its seats. Without this decision, Democrats believe they'd be two to four seats further away from a chance at the majority.

It was not obvious that the “zero tolerance” immigration policy, which separates undocumented immigrants from their children, would be a national political story. Merkley, a liberal with some presidential ambitions, broke the lock on the issue by filming his failed attempt to visit a detention center. In retrospect, Republicans see this as their worst campaign stretch of the year.

The president made a number of high-profile interventions in primaries, and this was, in some ways, a minor one; Trump endorsed Republican challenger Katie Arrington in a tweet just hours before the polls closed. But it was the only occasion in which Trump worked to beat an incumbent, and became part of a wave of successful endorsements. One thing they have in common: Many, like Arrington or Michigan gubernatorial nominee Bill Schuette, are having more trouble than the alternative candidates would have.

In the end, this was not a particularly disruptive year in party primaries; most of the movement came in races where members had retired. But the victory of Ocasio-Cortez over one of the top-ranking House Democrats turned her and her Democratic Socialist endorsers into national stars. That had an unexpected legacy on the rest of the year. It did not lead to more insurgent wins in federal primaries; almost everyone Ocasio-Cortez campaigned for lost, and just one more challenger, Massachusetts's Ayanna Pressley, knocked off an incumbent before the primaries were over. It did lead Republicans to prepare for more Democratic stumbles or shifts to the left in top races. They never came: Democratic ideological battles were largely contained to primaries in safe blue seats, such as the New York state Senate districts where insurgents would wipe out seven Democratic incumbents.

Did the conclusion of the Brett M. Kavanaugh hearings turn a fairly sleepy Supreme Court nomination into a galvanizing moment? Both parties say it did, disagreeing only about which of them it helped. The very first days after the hearings, however, reshaped Republican campaigns, with Graham leading the charge as the defender of a controversial nominee against a “mob" of rude and disruptive liberals.

October 27, 2018: The Pittsburgh massacre

The Parkland shootings had happened first, with long-ranging implications on how gun safety played as an election issue. But this solidified that trend; the president himself said that if there was Republican momentum in late October, this and the inert bombs sent to Democrats by a Trump supporter halted it.

The 2017 tax cut law simply never became a winning issue for Republicans, even after 11 months of solid economic growth. The president's 11th-hour float of a mysterious "10 percent tax cut" revealed what party polling had shown for months, that swing voters simply did not believe that the tax cut had benefited them. The decision by party leaders to punt “tax cuts 2.0" into next year was a punctuation mark on the election: It would be fought on Trump's terms, not House Speaker Paul D. Ryan's.

AD WATCH

National. Tom Steyer's “Need to Impeach" project has put on a different face for the midterms: It's running this “need to vote" ad on national cable, pivoting off Republican fulmination about the “angry mob" to encourage voters to turn out against Republicans. If you're watching cable in the final hours, you're seeing a lot this on CNN, and a lot of the super PAC ads warning of the caravan on Fox News.

California 25. One of the year's most striking ads is this Wes Anderson-esque spot for Democrat Katie Hill, who begins with a bland political backdrop that falls away to show her roaming her campaign office and tearing up a “corporate" check. Republican ads have tried to portray Hill as dishonest, using a quote from a town hall in which she said she wouldn't talk about “single payer" health care in the district. (She doesn't support it.) This is a more memorable spot, and a bit of jujitsu.

Maine 02. A major closing theme of this election has been Republicans abandoning economic conservative messaging to warn that liberal Democrats will attack Social Security and Medicare. That's the attempt in the NRCC's final ad against Jared Golden, which falsely accuses the Democrat of wanting to cut both programs; this follows a much-mocked ad for Rep. Bruce Poliquin (R-Maine), in which a doctor insisted that the Republicans' health-care plans wouldn't touch Medicare.

Michigan Senate. Even Republicans who think that Sen. Debbie Stabenow (D) will be reelected want to boost Republican challenger John James; the party has no obvious front-runner for what could be competitive 2020 races in the 11th District and against Sen. Gary Peters (R-Mich.). Ending Spending Action Fund, the Ricketts family PAC, made a late $1 million buy to advertise the “bold new leadership" James would bring; like Republicans' own ads this cycle, it doesn't mention the candidate's party or policies.

Nevada Senate. For the second time, Sen. Dean Heller (R) has enlisted Gov. Brian Sandoval (R), the state's most popular politician, to make the case for him. Unlike a prior ad, this doesn't touch on issues; it says mostly that Heller “gets the job done."

Pennsylvania 16. If this year is the inverse of 2010's tea party wave, this spot would demonstrate why; it goes after Rep. Mike Kelly (R), who won in that year as a car dealer with no political ties, for taking a “kickback" in the form of a tax change that helped car dealers.

POLL WATCH

Florida Governor (Quinnipiac, 1,142 Likely Voters)

Andrew Gillum (D) - 50%

Ron DeSantis (R) - 43%

This pollster has Sen. Bill Nelson (D-Fla.) up by the same margin and both Democratic nominees easily winning independent voters; if they do, both parties think the race is clearly over, especially after a strong Democratic finish in early voting. Both Democrats also lead narrowly in north Florida and the Panhandle, the part of the state that doomed Hillary Clinton's campaign in 2016 after a strong advantage for her in the early vote.

Missouri Senate (NBC/Marist, 600 Likely Voters)

Claire McCaskill (D) - 50%

Josh Hawley (R) - 47%

One month ago, Republicans suggested that the debate over the Supreme Court nomination of Brett M. Kavanaugh would put this race out of play for Democrats. Two weeks ago, they suggested that Hawley was breaking out, much as Rep. Kevin Cramer (R) had in North Dakota. And yet the president is spending his final evening before the campaign in Missouri's "bootheel." This poll puts the president's job approval in the state at +7, which Republicans think is low, but Hawley has had an awful final week, dominated by stories about whether he turned the attorney general's office into a political operation for this race.

New Hampshire Governor (UNH, 630 Likely Voters)

Chris Sununu (R) - 49%

Molly Kelly (D) - 49%

Democrats have talked up their chances here ever since a surprisingly high-turnout primary on September 11, but this is the first and only poll to show first-term Gov. Sununu in a dogfight. This is a state where elections can and do break in one direction in the final days; four years ago, then-Gov. Maggie Hassan saw a double-digit lead collapse in the last week and won by five points. While Democrats want to beat Sununu, any evidence that they can shore up the state's House seats and gain (or win) the General Court. (That's what New Hampshire calls its legislature. Get used to it, because Democrats will be spending much of 2019 up here.)

New Jersey Senate (Quinnipiac, 1,115 Likely Voters)

Bob Menendez (D) - 55%

Bob Hugin (R) - 40%

After panicking in public about this race and putting close than $10 million into TV ads reminding voters that Hugin donated to Trump, Democrats think they're ahead here; perhaps not as much as this poll suggests, but for the same reasons. Independent voters, the target of millions of dollars in Hugin ads, are seen to be breaking against the president's party. Some Republicans have started to compare this to the 2014 battle in Kansas, when Independent candidate Greg Orman, backed by Democrats, could not survive the connection to a president who was toxic in his state.

2020

Cory Booker. His home state just passed a law that would allow him to run for president in 2020 without abandoning his Senate seat. The risks to Democrats of a Booker run plummeted in 2017 anyway; through 2021, it's up to Gov. Phil Murphy (D-N.J.) to appoint a replacement and schedule an election if there's a vacancy.

WHAT I'M WATCHING

Everyone who walks the plank and makes election odds has now weighed in; universally, the prognosticators see a Democratic House and are skeptical that the closest Senate races will break for Republicans.

Larry Sabato's Crystal Ball

House: Dems + 30

Senate: GOP + 1

Governors: Dems + 10​

National Review

House: Dems + 32

Senate: GOP + 1

Governors: Dems + 9

FiveThirtyEight*

House: Dems +39

Senate: Dems +1

Governors: Dems +9

The asterisk is there because, as Nate Silver is happy to point out on Twitter, FiveThirtyEight does not make race predictions. It calculates odds, and at the end of Monday it gave Democrats in the closest races an almost marginally better chance of victory.

How did these analyses fare in 2016? All of them slightly underrated Republicans.

READING LIST

Nobody would be more shocked than Democrats if Schumer wakes up Wednesday as the leader of a majority. The story here is more about how Schumer handled the past two years, letting his conference roam on many votes but knotting it together on health care and taxes.

A deep dive into the new technology, built since 2016, that insurgents are using in the hopes of defying history.

An amusing catalogue of complaints from candidates who are pretty tired of seeing incumbents tout their membership in a caucus that has only made a couple of deals.

COUNTDOWN