(CNN) First things first: The theme song of the week is Theme from Mission: Impossible from the television show Mission: Impossible.

It's the first non-partisan poll of the race that uses live interviews and called cell phones. There have been 16 incumbents who were down in the polls at this point in the cycle since 2006. Six of them (38%) won.

What's the big idea: The new Mason-Dixon poll is bad news for Democrats who probably need to hold onto the North Dakota Senate seat in order to have a realistic shot of winning back control of the Senate in 2018.

Polls indicate that the national environment will favor Democrats this fall. If every Senate seat were up for reelection, Democrats might have a good shot of winning a majority. But only about a third (35) actually have elections this fall.

Democrats currently control 26 of these Senate seats. That means there are only nine Republican held seats from which Democrats will need to win at least two to have a majority come next January.

For every Democratic held Senate seat the Republicans win, the Democrats need to win an additional Republican seat. So if Republicans win in North Dakota (and no other Democratic held seat), then Democrats will need to pick up three Republican seats.

The problem is that Democrats aren't competitive in many of the nine Republican-held seats. Democrats are massive underdogs in Mississippi regular (i.e. non-special election), Nebraska, Utah and Wyoming. That leaves just five seats from which Democrats would need to win three, and a number of those will be tough hills to climb.

Of these five, Democrat Hillary Clinton only won Nevada. Early polls suggest Republican Sen. Dean Heller is in a tight race against Democrat Jacky Rosen.

The only other state that was close in 2016 among the nine was Arizona. Sen. Jeff Flake is retiring and polls indicate that Democrat Kyrsten Sinema is ahead of likely Republican opponent Martha McSally.

But even if Democrats win these two seats, a loss in North Dakota means that they have to win one more Republican seat. The most obvious suspect would be Tennessee, where former Gov. Phil Bredesen has a small lead in early polls over Republican Marsha Blackburn. Tennessee though, voted for Republican President Donald Trump by 26 points, and there has been a tendency for races to revert back to their partisan leanings when the early polls show something different.

The other two seats are much longer-shots for Democrats. In the Mississippi special Senate election, Democrats would need for Democrat Mike Espy and Republican Chris McDaniel to advance to a runoff after the contest's jungle primary. Early polls have Republican Sen. Cindy Hyde-Smith advancing to the runoff with Espy instead. And even if the Democrats preferred runoff occurred, keep in mind that Trump won Mississippi by 18 points and no Democrat has won a Senate race there since 1982.

Or Democrats could try to win in Texas. Republican Sen. Ted Cruz has a clear lead over Democrat Beto O'Rourke in this state that hasn't voted for a Democratic presidential nominee since 1976 or voted a Democrat into the Senate since 1988.

But let's say that Democrats turn the table and win three of these five seats. They'd still need to hold onto all 25 of the other seats that they control heading into the election. Most of those will be lay-ups, though a few definitely won't be. Besides North Dakota, there are four other Democratic-held seats with Senate races in states that Trump won by about 20 points or more: Indiana, Missouri, Montana and West Virginia.

Although the latest public polling has Democrats ahead in Montana and West Virginia, it's far less clear what is occurring in Indiana and Missouri. Polls also show a close race in Florida. A Democratic loss in any of these and North Dakota means that Democrats need to win four Republican held seats. A loss in North Dakota and two of them means they have to win five. A loss in North Dakota and three of these other seats means Democrats have to win six Republican held seats, which seems highly unlikely.

The bottom line is if Democrats don't win the North Dakota Senate race the path to a Senate majority in 2019 becomes greatly complicated and near impossible if they lose another seat they currently hold.