Manchin's surprise decision to stay in the Senate and, presumably, to seek a second term in 2018 is a major break for Democrats who badly needed one. Had Manchin abandoned his seat, there is a roughly zero percent chance Democrats would have held it in November 2018. Manchin is a popular former governor who is regarded as apolitical, a necessary status in a state that Donald Trump won by 42(!) points in 2016. Even with his decision to run again, Manchin could face a real challenge from Reps. Evan Jenkins or Dave McKinley or state Attorney General Patrick Morrissey. But the incumbent Democrat will start a race against any one of that trio as a favorite.

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Even with Manchin a somewhat surprising “yes” for reelection, the math is incredibly daunting for Senate Democrats.

Start here: Democrats have to defend 25 seats in 2018 as compared with just eight for Republicans. Of those 25, 10 — Florida, Indiana, Michigan, Missouri, Montana, Ohio, Pennsylvania, North Dakota, West Virginia and Wisconsin — were ALL carried by Trump in November. Five of those states — Indiana, Missouri, Montana, North Dakota and West Virginia — were also carried by Mitt Romney in his 2012 loss to President Obama. Trump's margin of victory in those five states was a whopping 27.5 points.

Manchin's decision gives Democrats a fighting chance in one of those five. The next question is whether North Dakota Sen. Heidi Heitkamp stays or goes. She is widely mentioned as a top candidate for agriculture secretary in a Trump administration although it remains unclear if she would accept the job. If Heitkamp did leave the Senate for Trump's Cabinet, her seat would be a near-certain pickup, with Rep. Kevin Cramer (R) already being recruited to run.

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Even if Democrats keep Manchin and Heitkamp in the fold, however, they are still going to need a LOT of breaks over the next 23 months to avoid widespread losses. Just how bad is the 2018 map for Democrats? This, from Kyle Kondik and Geoffrey Skelley at the University of Virginia's Center for Politics, captures the predicament well:

Only three times before in the era of popularly-elected senators has a party begun a midterm cycle as exposed as the Democrats are in 2018, and only once since World War II. When we say “begun,” we mean the partisan makeup of the class up for election in December about two years prior to Election Day — where we are in the calendar right now.

The best Democrats can hope for, Kondik and Skelley write, is a repeat of 1970, when Democrats managed to hold 11 of the 12 seats in states Richard Nixon had carried two years earlier. Republicans netted only three seats that year.

To have any chance at lessening their losses in two years' time, Democrats must limit the number of open seats they have to defend. Manchin appears set now. Heitkamp remains very much a question mark. Joe Donnelly in Indiana, Claire McCaskill in Missouri and Jon Tester in Montana have all said they will run for reelection despite the challenging political environments in their states. The big question mark looks to be Sen. Bill Nelson of Florida, who insists he will run for another term, but at age 74 may reconsider — particularly if it looks like he will face a spirited race in the expensive Sunshine State.

Another major question is whether the split between Trump and the Republican establishment made clear in this election plays itself out in GOP Senate primaries around the country in 2018. Republicans clearly hamstrung themselves in 2010 and 2012 when the tea party wing pushed unelectable nominees such as Christine O'Donnell (Del.) and Todd Akin (Mo.) into general elections over more establishment figures with track records of cross-party appeal.

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And then there is the weight of history, which suggests Democrats are likely to have something of a wind at their back in the first midterm election of President Trump. In President Obama's first midterm in 2010, Democrats lost 63 House seats and six Senate seats. In his second, Democrats lost 13 House seats and nine Senate seats. The average loss of the president's party in midterm elections since 1982 is 20 House seats and five Senate seats.