And if you look over his list of districts, you start to get a sense of why. To get to the magic number — a 30-seat gain — Democrats will need to wade into some very red districts and win with some not-very-highly touted candidates. Oh, and they'll need to basically run the table.

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I encourage you to read Gersh's breakdown over at the National Committee for an Effective Congress's website. Below, we point to some of the Democrats' biggest challenges/obstacles:

1. They need to go into very rough territory — rougher than the GOP did in 2014

As Gersh points out, a 12 to 15 seat gain is quite possible. Democrats basically just need to win territory that President Obama won and some very winnable open seats.

The terrain beyond that is where things become much trickier.

Gersh concedes that Republicans will likely win a couple of Democratic seats and then lays out 35 winnable districts, of which Democrats must win almost all to get their 30-set gain.

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And as you venture down his list, you'll notice some ruby-red districts. Rep. Kevin Yoder's (R-Kan.) district, which went for Mitt Romney by 10 points in 2012, is one of them. Romney also won the congressional district being vacated by Senate candidate Rep. Todd C. Young (R-Ind.) by 16, and the district of Rep. Ron DeSantis (R-Fla.) by 17.

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In fact, Democrats would have to win in significantly tougher territory than Republicans won in their 2014 wave election, which gave them their biggest majority in many decades.

The Cook partisan voting index (PVI) rates each district according to whether it is more Republican or Democratic than the nation as a whole. Republicans in their 2014 wave election took over just four districts that had a Democratic-leaning PVI: Rep. Rod Blum of Iowa (D+5), Rep. Bruce Poliquin of Maine (D+2), Rep. John Katko of New York (D+5) and Rep. Cresent Hardy of Nevada (D+4).

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Gersh's list of 35 winnable districts, though, includes 16 that lean Republican, according to the Cook PVI — four times as many. Yoder's, Young's and DeSantis's districts are among the toughest, but they have plenty of company.

2. Democrats can't afford setbacks, but setbacks happen

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DeSantis's seat is a good example of how quickly a seat can come off the map for reasons that are beyond national Democrats' control.

It was an open seat until Sen. Marco Rubio (R-Fla.) opted to run for reelection two months ago. DeSantis, who had been seeking Rubio's Senate seat, went back to running for reelection. He's got a primary challenger, but if he wins the primary, it's hard to see Democrats putting up much of a fight there — especially with a field of challengers who haven't raised much money.

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Similarly, Democrats lost a potential pickup opportunity earlier this month in Washington, when a candidate who had dropped out of the Democratic primary nonetheless won the nomination to face Rep. Dave Reichert (R-Wash.). The "zombie candidate" won't actually run for the seat, and Reichert appears safe.

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And back in April, the Democrats' hand-picked candidate for a winnable seat in the Philadelphia suburbs held by Rep. Pat Meehan (R-Pa.) also lost his primary — by more than 50 points.

All three of these seats are lower-tier Democratic targets, but are still the kind of seats they'd want to put in-play to have a better shot at 30.

Another primary worth watching: the Democratic Congressional Campaign Committee would love to have Annette Taddeo-Goldstein beat scandal-tarred former congressman Joe Garcia in the race to face freshman Rep. Carlos Curbelo (R-Fla.) -- one of the Democrats' best pickup opportunities on the map. Garcia lost a very close race against Curbelo in 2014.

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And again, Democrats need the vast majority of things like these to go right.

3. Some Democratic candidates have very little money

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Part of the problem Democrats face is that Trump locked up the GOP nomination after it was too late to recruit in many states. That means they are relying on some candidates that aren't top-tier or even second-tier recruits. Accordingly, some of their candidates have almost no money.

Rep. Edward R. Royce (R-Calif.) has an opponent that had less than $4,000 in his campaign account at the end of June. The opponent of Rep. David Valadao (R-Calif.) had less than $60,000. Yoder's had about $74,000. All three are on Gersh's list.

Those are pittances in modern politics and suggest these candidates will need to be swept up in a wave because they won't be able to advertise much. Yes, it's happened before, but money matters in politics too.