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CNN  — 

Democrats are holding onto the US Senate by a thread. The chamber is tied 50-50 with Vice President Kamala Harris breaking ties in the Democrats’ favor. Republicans need to pick up only one seat in next year’s midterm elections to gain control of the chamber, and an early look at the 2022 Senate playing field suggests they should be considered favorites.

I designed a model that takes into account experts’ race ratings, the last presidential vote in each state, whether the incumbent is running for reelection, and the generic congressional ballot. I did that for the 2022 cycle and examined how these factors were correlated with Senate election outcomes since 2006. Based on those calculations, the GOP’s odds are roughly 3 in 4 (75%) to pick up one seat for a majority. The most likely outcome is a Republican net gain of two to three seats.

To be clear, there’s a lot of uncertainty in this estimate, which reflects a range of probabilities to account for slightly different ways of projecting outcomes of races. In fact, depending on how things go over the next year, anything from a sizable Democratic gain of greater than five seats to a Republican gain of greater than five seats is possible.

The takeaway, though, is that Republicans more often than not will have a net gain, and therefore take control.

The reason I say Republicans are favored is because of the seats that are up for grabs and what history tells us about how the elections in those seats will play out.

Republicans are seen as the favorites by the model for a simple reason: the GOP advantage in the average generic congressional ballot poll. A Republican advantage on the generic ballot this early is unusual, and it hasn’t happened since at least 2006.

However, the map this cycle is not one where the Republican advantage is apparent at first glance. Not only are there more Republican-held seats up for grabs (20 to the Democrats’ 14), but there are slightly more Republican than Democratic-held seats up for reelection in 2022 in states that were decided by less than 10 points in the 2020 presidential election.

Race ratings also indicate something closer to a tossup in the fight for the Senate.

Historically, though, it takes time for the microenvironment (i.e. individual seat designations) to catch up to what the macroenvironment (i.e. generic congressional ballot) indicates will happen. Moreover, we know from past years that the generic ballot at this point in midterm cycles tends to underestimate the tailwinds the opposition party will have at their backs come election time.

You can see these phenomena by looking at the Cook Political Report and Crystal Ball ratings. Right now, Cook and the Crystal Ball have three Democratic senators (Mark Kelly in Arizona, Raphael Warnock in Georgia and Catherine Cortez Masto in Nevada) in races rated as tossups. They also have the GOP-held seat in Pennsylvania, where Sen. Pat Toomey is retiring, as a tossup. Both have New Hampshire as leaning Democratic. President Joe Biden won all of these states by 7 points or less last year.

If trends in past years hold, the races designated as tossups are more likely to lean Republican come election time. In other words, Republicans are projected to win those states about 7 in 10 times. This makes sense given the fact that Biden barely won these tossup state in 2020 and 2022 is shaping up to be a more pro-Republican year.

In New Hampshire, which Biden won by about 7 points, the model suggests the race is actually a tossup.

Cook rates North Carolina, which is an open seat, and Wisconsin, where GOP Sen. Ron Johnson may or may not run for another term, as tossups, while the Crystal Ball rates them as leaning Republican.

As in other tossup states (save Nevada), these states were decided by about a point or less in the 2020 presidential election. With the pro-GOP national environment, Republicans in both states are favored by the model to win about 4-in-5 times.

Now, it’s important to keep in mind that Republicans may not win all or even any of these seats. Republicans could even lose seats where they have better odds but that experts rate as competitive, albeit leaning or likely to be won by a Republican, like Florida (where Sen. Marco Rubio is running) or Ohio (an open seat).

The key thing, though, is that Republicans have a better chance in the seats they’re looking to pick up than Democrats have in the seats they’re looking to pick up. When you start adding those probabilities across races, it’s likely that Republicans will win the seats necessary to secure the majority, even if they lose one unexpectedly.

Moreover, lopsided national environments can produce surprising results in unexpected places. Twelve years ago at this point, the Wisconsin Senate race was rated solid Democratic by the Cook Political Report. Such a rating made sense: Democrats had easily carried the state on the presidential level in 2008 and Democratic Sen. Russ Feingold was running for reelection. Feingold ended up losing, however.

Outside of perhaps Colorado, where Democratic Sen. Michael Bennet is running for reelection next year, I’m not sure you could point to one state where such a surprise result might happen. But if past years hold, there’s probably going to be one state that’s not on people’s radar that will eventually become competitive.

The model takes that into account, and it gives Republicans an extra boost.

The bottom line here is pretty simple: Republicans have a better shot than Democrats of having a Senate majority after 2022. It’s not a foregone conclusion, however, with a lot of campaigning to go.