A first look at the 2020 Senate map: Republicans have a good shot at maintaining control

(CNN)Voter enthusiasm for the 2020 presidential election is already sky high. With all the coverage of Beto O'Rourke's counter climbing and Pete Buttigieg's speaking approximately 200 languages, you might forget that 34 Senate seats are also up for election. You can bet those races will soon be hotter than a firecracker, given the Senate's power to confirm Cabinet members and federal judges -- such as the justices who sit on the Supreme Court.

Republicans have a better shot than not of maintaining control of the Senate in 2020. However, recent trends indicate that the possibility of a clean sweep for either side (i.e. taking the presidency and the Senate) is higher than it's ever been.
Democrats need only a net gain of three Senate seats to win control, if they win the presidency. In such a case, the vice president would break the tie. With Republicans controlling 22 of the 34 seats up for election, Democrats have, in theory, a wide playing field from which to choose.
The problem for Democrats is that the biggest trend in Senate elections is straight ticket voting. When you cast a ballot for one party on one part of the ticket in recent elections, the chance of voting for that same party on other parts of the ticket is significantly higher than it used to be.
    An astounding 87% of the differences in the Senate margins across states could be explained by the statewide aggregate House margins in 2018. Ergo, if you voted for a Democratic (Republican) candidate for the Senate last year, there was a very high likelihood that you voted for a Democratic (Republican) candidate for the House.
    The same pattern is apparent when comparing the Senate results last year with past presidential election results across states. States that leaned Democratic (Republican) in past presidential elections were far more likely to vote Democratic (Republican) in Senate races last year. The Senate and last two presidential election results were more simpatico in 2018 than Senate and past presidential results had been in any midterm election since at least 1982.
    This comes on the heels of 2016, when every state that sent a Republican to the Senate voted for Republican Donald Trump and every state that sent a Democrat to the Senate voted for Democrat Hillary Clinton. In no other presidential year in the last 100 had this phenomenon (of all states voting the same for Senate and presidency) occurred.
    So how does that shake out for the 2020 map?
    Of the 22 seats controlled by Republicans, just two of them are in states (Colorado and Maine) that leaned Democratic compared with the nation as a whole in a weighted average of the last two presidential elections (75% in 2016 and 25% in 2012). Today, President Trump is quite unpopular in both. Additionally, both states were very Democratic in the aggregate House vote in 2018.
    Now, they certainly aren't gimmes, given that both states were within 5 points in the last presidential election. Still, Democrats have clear pickup opportunities in Colorado and Maine, for the seats now held by Sens. Cory Gardner and Susan Collins, respectively.
    Therefore, you would think that Democrats were just one seat away, if things went well in Colorado and Maine.
    Here's the complication: Alabama. You may recall when Democrat Doug Jones shocked political analysts and pundits by beating very flawed Republican Roy Moore in a 2017 special election. Now that seat is up again. The good news for Democrats is that Moore, who faced sexual abuse allegations and had an archconservative record, may be running again. But even if Moore won the nomination (and chances are he won't), Jones wouldn't benefit from depressed Republican turnout this time around, because Trump will be on the ballot.
    Alabama is a state Trump won by nearly 30 points and remains popular in. Republicans won the aggregate House vote there in 2018 by more than 20 points. That means Jones has an uphill climb for re-election by the numbers.
    In other words, even if Democrats pick up Colorado and Maine, they'd probably be looking at a net gain of one.
    Beyond these seats, the Democratic pickup opportunities slim dramatically. Of the other 20 Republican-held seats up for election, 16 of them are in states that were 10 points or more Republican than the nation as a whole in a weighted average of the last two presidential elections. None of these races look competitive at this time.
    The other four have leaned 5 to 10 points more Republican than the nation in a weighted average of the last two presidential elections: Arizona (Martha McSally), Georgia (David Perdue), Iowa (Joni Ernst) and North Carolina (Thom Tillis).
    It's conceivable that Democrats could win in these states. They just won a Senate election in Arizona against McSally, who was appointed to her Senate seat following her defeat in the race for the other one. The statewide 2018 House votes in all these states were close. The best Republican state was Georgia, which Republicans won by less than 2 points in the aggregate House vote.
    Conceivable doesn't mean likely, however. Elected Republican incumbents are, at this point, expected to be running for all these seats, except for Arizona. Generally, incumbents tend to do better than non-incumbents. Even if the 2018 political environment were in effect (i.e. one where they won the national House vote by high single digits), the lean of each state in the 2018 House elections suggests that only Arizona (because McSally wasn't elected) would go to the Democrats.
    So even with what would be a really good political environment for Democrats in effect, the most likely outcome is that Democrats would fall short of the net gain of three they need to take control of the Senate.
    Still, it would be a competitive race for Senate control if the 2020 political environment matched the 2018 environment. With good candidate recruitment or just flat-out luck, Democrats could win one or more seats they would otherwise be expected to lose by a small margin. In turn, they would be in a strong position to gain Senate control if a few things went their way.
    Also it's quite plausible the national environment is better for Democrats, given that the President remains quite unpopular.
    There's no way of telling, of course, if the political environment will be as good for Democrats in 2020 as it was in 2018. There's a decent chance Trump could become less unpopular or that Democrats have their own unpopular nominee for president.
    If the national political environment shifted closer to neutral, the odds for a Senate Democratic majority would drop dramatically. Democrats would not only have a difficult time flipping Arizona, Georgia, Iowa or North Carolina, but Republicans would have a good shot of holding on in Colorado and Maine. Further, there are a number of Democratic-controlled seats that could be put into play. Michigan (Gary Peters), Minnesota (Tina Smith) and New Hampshire (Jeanne Shaheen) are all swing states (e.g. voted within 1 point of the nation in a weighted average of the 2012 and 2016 presidential votes).
    These three Democratic-held seats aren't expected to be all that competitive, given that Democratic incumbents are running in them and the political environment is still pretty bad for Republicans. For reference, Democrats won the aggregate House vote in all these states by more than 5 points in 2018. It's not difficult to see these races being tight, however, if the national environment improves for Republicans.
    To see how things could go poorly for Democrats if the national environment shifts, look at Michigan. Last year, Democratic Sen. Debbie Stabenow won by less than 7 points. That is, she actually won by less than Democrats won by in the national House vote (a good gauge for the national political environment). If the national political environment shifts closer to neutral in 2020, it's not difficult seeing next year's Michigan US Senate race being quite competitive.
      The good news for Democrats is that beyond Alabama and these three Democratic-held seats, there really aren't any that Republicans look like they will put into play at this time. That means Republicans probably won't be able to come close to a filibuster-proof (60 votes) Senate majority even in a good year for them.
      The bottom line: Republicans are favored to maintain control in the Senate, but keep an eye on the presidential race to know how the Senate races go. The chance of the victor winning all the spoils is higher than it's been in many years.