President Trump meets with Senate Majority Leader Mitch McConnell (R-Ky.), Senate Minority Leader Charles E. Schumer (D-N.Y.), House Minority Leader Nancy Pelosi (D-Calif.) and other congressional leaders in the Oval Office in September. (Kevin Lamarque/Reuters)

The possibility that Democrats could win the Senate in 2018 seemed preposterous a year ago, given President Trump's stunning victory and the basic math facing a party defending three times as many seats as Republicans in November's midterm elections.

Not anymore. The debate has grown over Democrats' chances of capturing control of the agenda and holding power over Trump's nominations, including potential vacancies on the Supreme Court.

The dispute pits the practitioners of big data against those who also scout candidates and measure broader political atmosphere to make their bets.

Both sides agree Democrats face a narrow path to gain two seats needed to reclaim the majority — but there is a debate over just how narrow.

Even after Sen. Doug Jones (D) won his improbable Alabama special election last month, Democrats still face an imposing task: They are defending 26 seats of their own to just eight Republican seats up for grabs.

"Just how bad is this map for Democrats? It's bad enough that it may be the worst Senate map that any party has faced ever," Nate Silver, founder of Five Thirty Eight, the data analytics blog covering sports and politics, wrote Wednesday.

Sen. Heidi Heitkamp (D-N.D.) will not face a challenge from North Dakota state Rep. Kevin Kramer, depriving Republicans of the candidate many saw as the party's best chance to unseat Heitkamp. (Manuel Balce Ceneta/AP)

But some veterans suggest that the broader national environment is beginning to break so sharply against Trump and Republicans that the Senate could very much be in play.

"A key question for November is which will be dominant: the environment or geography. Put another way, in both the House and Senate, it's the wave versus the map," Charlie Cook, the founder of the Cook Political Report, retorted a day later.

Even before Trump took office, members of both parties agreed that the House majority would be up for grabs. On a broad level, the 2016 presidential math looked relatively stable — Trump won 230 House districts, almost exactly the same as Mitt Romney in 2012 — but there was a dramatic shift at the ground level. Several dozen suburban districts swung into Democrat Hillary Clinton's column or went from strong Romney districts to narrow wins for Trump. Rural districts, meanwhile, broke far out of reach for Democrats.

Both Silver and Cook believe the Democratic case for picking up the 24 House seats they need for the majority has only gotten stronger amid Trump's historic unpopularity for a first-year president. Silver actually believes some analysts have been "slow to recognize just how bad things had gotten for Republicans," given election data from 2017 congressional special elections and Virginia's gubernatorial race.

Cook believes Democrats are now favored to win the House.

"Just about any fair-minded assessment of the House would show that Democrats have probably more than a 50-50 chance of taking control of the House," he wrote.

The Senate has always been a steeper climb for Democrats. Those 26 seats Democrats are defending include five in states that Trump won by more than 18 percentage points — plus five more in states he won by smaller margins. Sen. Dean Heller (Nev.) is the only Republican running in a state that Clinton won.

Look at North Dakota. In 2012, Republican Romney won comfortably over then President Barack Obama by more than 63,000 votes, 58 percent to 39 percent. But Sen. Heidi Heitkamp (D-N.D.) ran a strong enough campaign to eke out a 3,000-vote win. She won 36,000 more votes than Obama — meaning there were lots of Romney-Heitkamp voters.

By 2016, Democratic support had collapsed in Heitkamp's state. Clinton won just 27 percent of the vote, losing by more than 120,000 votes to Trump.

If North Dakota has turned this Republican, it seemed, any generic Republican would topple Heitkamp this year.

That's also the situation Sen. Joe Manchin III (D) faces in West Virginia, where Clinton won just 26 percent of the vote. Democrats are also defending three more states — Indiana, Missouri and Montana — where she received less than 40 percent of the vote.

Add into that mix another five seats in Trump states, particularly Florida, where Sen. Bill Nelson (D), at 75, is preparing for a potential challenge from a candidate who would be his toughest opponent ever: Gov. Rick Scott (R).

If the Democrats lose two or three of those races — which a year ago would have been a conservative guess — the majority is likely to be simply out of reach. Beyond Nevada, conventional wisdom offers only one other pickup opportunity: the seat of retiring Sen. Jeff Flake (R-Ariz.). Senate Minority Leader Charles E. Schumer (D-N.Y.) recruited Tennessee's popular former governor, Phil Bredesen, to come out of retirement, but it remains a deep-red seat and it's unclear if Bredesen has modern campaign skills.

Silver argues that the most likely Democratic path to the majority is a surprising new opening, for instance if Sen. John McCain (R-Ariz.), who is battling brain cancer, is unable to complete his term or if a fresh scandal erupts among one of the other Republican incumbents from more conservative states.

In all, he gives Democrats a 35 percent chance of winning the Senate. "It's higher than it probably 'should' be given how favorable the Senate map is for Republicans. But it's still a fairly steep hill to climb," Silver wrote.

Cook agrees with these factors, but he compares himself to a baseball scout "watching the numbers but taking a lot of nonnumerical points into consideration as well."

Last week started with retirements from veteran California Republican Reps. Edward R. Royce and Darrell Issa, followed by the decision of Rep. Kevin Cramer (R) not to challenge Heitkamp in the Senate race.

When it comes to Senate races, they almost all break in one direction. According to Jennifer Duffy, Cook's Senate race expert, one party has won at least two-thirds of the toss-up races since 1998. In 2016, Republicans won five of seven races the Cook report rated as toss-ups. In 2012, Democrats won eight of 10 toss-ups.

Democrats will need a performance similar to that of 2012 to get over the hump to win the majority, but Cook is beginning to sound like that grizzled scout trying to let management know that he has found a special pitcher.

"The closer we get to the election, the better sense we'll have of whether the wave or the map is winning out," he wrote.

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