(CNN) There are 35 Senate seats on the ballot across the country this fall. But if you are trying to figure out whether Democrats have any plausible chance at winning back the majority, there's one race that should tell you all you need to know: the Tennessee Senate race.

Tennessee is one of a quartet of Republican-held seats where polling suggests that Democrats have a real chance of winning -- sharing that distinction with Arizona, Nevada and Texas. But those four seats are not created equal: Nevada and Arizona look like they are leaning slightly toward Democrats, while Texas, due to its partisan lean, is the toughest of the four for the minority party to win.

Remember too, the math: Democrats need to net two seats in order to seize control of the Senate. And Democrats have to defend 26 seats -- including 10 in states President Donald Trump won in 2016. If Democrats won Arizona and Nevada -- which, as of right now, is totally plausible -- the party would have to hold all of its own seats, including in strongly pro-Trump states like North Dakota, Indiana and Missouri. While Democrats insist they feel generally good about that trio of states, it's easy to imagine that at least one of those three Democratic incumbents loses -- most likely Sen. Heidi Heitkamp. Which would mean that Democrats would net only a single seat and the chamber would be knotted 50-50, with Vice President Mike Pence breaking the ties. And that would mean Republicans still, at the end of the day, controlled the chamber.

Which brings me back to Tennessee, and a brand-new poll from CNN that shows former Tennessee Phil Bredesen (D) leading Republican Rep. Marsha Blackburn (R) 50% to 45%. That spread, if it holds, would give Democrats a cushion the party desperately needs to have any margin for error. Now, rather than needing to run the table among their own seats -- which, even in this national environment, is a very hard thing to do -- they could afford to lose, say, Heitkamp and still have a shot at the majority.

Yes, you could make a similar argument about Texas and Rep. Beto O'Rourke's campaign against Sen. Ted Cruz. But, compare the polling in the two states and you see that, at least at the moment, the two races are not equal opportunities.

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