National Journal has just published a new batch of Senate race rankings, and the overall picture they paint looks very grim for Democrats. I’m not qualified to do the deep numbers-crunching prognosticating, so consider this a rough back-of-the-envelope look. Using National Journal’s rankings as a jumping off point, it’s worth breaking the races down into four categories:

1) Races that Republicans are pretty much certain to win: South Dakota and West Virginia (which is not absolutely certain but very likely). That’s probably two seats to Republicans at the outset.

2) The five core red state battlegrounds: Montana, Arkansas, North Carolina, Louisiana, Alaska. These are races that Republicans have a very plausible chance at winning, but they require dislodging incumbents, so for now, they are fundamentally uncertain. The only certainty is that all will be long, hard slogs.

As Ron Brownstein has detailed, this is ground zero for understanding our current political reality: Control of the Senate will be decided in states that were carried by Mitt Romney in 2012, and are older and whiter than the broader coalition that Dems are increasingly relying on to win national elections. This, in a non-presidential year. However, in one or several or even more of these, Dems could hang on if conditions are right.

3) Races that will show Republicans have broadened the map if they remain very close later this year: Michigan, Colorado, Iowa, Minnesota. These races will certainly be competitive until the end, but it remains to be seen how competitive they are. In these races, Dems might win by high single digits, which would mean they had to run real campaigns to win but were never in serious danger.

Is there a scenario under which these races remain seriously in play after Dems engage? It’s too early to say, but it’s certainly possible, because they are swing states in which outside groups will spend enormous sums of cash.

4) Races where Dems have a shot at pickups: Kentucky and Georgia.

With the above in mind, here are a few possible scenarios:

1) The best case scenario for Dems: Republicans fail to seriously broaden the map, meaning the races in category three above aren’t meaningfully within reach, even as Dems manage to pick off either Georgia or Kentucky. If that happens, Republicans would have to sweep all five races in category two — the core battlegrounds – to win the Senate.

2) The non-awful, but somewhat difficult scenario for Dems: Republicans fail to seriously broaden the map into category three, but Dems fail to pick up either Georgia or Kentucky. If this happens, Republicans would have to sweep four out of five races in category two — the core battlegrounds — to win the Senate. That’s hard, but not as hard as Number One above.

3) The harrowing scenario for Dems: Republicans broaden the map even as Dems fail to win either Georgia or Kentucky. Given that Republicans start with two seats likely in their pocket, this would mean Republicans would have to win four out of seven, eight, or even nine races from categories two and three to take the Senate. This is certainly plausible.

One caveat: Dems say they are in the worst moment in the cycle right now, with Obamacare’s rollout problems only just fading from the news, and outside GOP-aligned groups pouring huge money into areas where Dem campaigns are not spending resources yet. At some point those resources — which are currently being stockpiled and will be considerable — will be unleashed.

There are all kinds of unknowns that will influence which scenario above is most likely, such as the state of the economy and external events. To make things more unpredictable, there are all kinds of unknowns that will influence how things unfold in individual races within whichever larger scenario takes hold — factors like the candidates, their campaigns, and local issues.

Bottom line: Dems are very much on defense, but that’s largely due to the fundamentals, which turn on the fact that the race will be decided in a core battleground made up of red states. A GOP takeover is very possible. But it’s way too early to say with any certainty how all of this will play out, because so much is in flux.

Greg Sargent writes The Plum Line blog, a reported opinion blog with a liberal slant -- what you might call “opinionated reporting” from the left.