(CNN)The pending Senate battle over the next Supreme Court nominee shows us how important winning Congress’ upper chamber is this fall. A president who doesn’t have the Senate backing him cannot fully use the power of the presidency to influence the government’s direction.
2020 could be shaping up to be a winner-take-all situation. Given the lineup of seats up in the Senate, chances are Democrats will take the Senate if former vice president Joe Biden takes the presidency. If President Donald Trump wins a second term, chances are Republicans will retain control of the Senate. Right now, the Democrats are favored to take control of the Senate because of Biden. We could be looking at record straight ticket voting this cycle, and Biden’s advantage in the presidential race is seeping down into the Senate races. The Democrats need a net gain of three to four seats to win the Senate: three if Biden wins and Kamala Harris, as vice president, breaks a 50-50 split, and four if Biden loses and Vice President Mike Pence breaks a tie. Chances are Democrats will only need a net gain of 3 because Biden’s ahead in the presidential race. And given what we’re about to talk about, it’s unlikely Democrats will get a net gain of 4 seats if President Donald Trump wins another term. Read MoreDemocratic candidates for Senate have at least nominal polling advantages in five seats currently held by Republicans. They are Arizona, Colorado, Iowa, Maine and North Carolina. At the same time, Republicans hold a polling lead in one Senate seat currently held by the Democrats: Alabama. Not surprisingly, Biden is doing much better in the five states where the Democrats look best positioned to pick up Senate seats than he is in Alabama. In fact, he is ahead in all five of those states, save maybe Iowa where different polling averages disagree on who the leader is. Either way, Biden’s position is weakest in Iowa where the Democratic Senate candidate (Theresa Greenfield) has the smallest lead of the five current seats Democrats look best positioned to pick up. All told, with maybe the exception of Iowa, every single state where Biden is leading also has the Democratic Senate candidate ahead. Likewise, every single state where Trump is ahead also has the Republican candidate for Senate leading. There are potential exceptions to this rule. Biden is clearly behind in places such as Kansas, Montana and South Carolina. The Democratic Senate candidates, though, could win in any of them. Right now, however, they’re slightly behind. However, it’s hard to imagine Democrats winning in those races if Biden loses the presidency. This could be the second presidential election in a row and only the second since senators were first popularly elected where every state votes for the candidate of the same party in both the presidential and Senate race. The 2020 election could even top the 2016 election in terms of the high correlation between presidential and Senate voting patterns in a state. I took the FiveThirtyEight presidential polling average in every state and compared it to the FiveThirtyEight Senate lite model in those states (except for the Arkansas Senate race where no Democrat is running and the Georgia special Senate election where multiple candidates from each party are running). Both of these measures are essentially poll averages that are adjusted for the trendline in other states when there is a lack of recent polling in a given state.In total, we’re looking at 28 Senate races. The average difference between the presidential and Senate margins in all the races is about 4.3 points. If we’re looking at both the Senate races at the 14 races within 10 points, which tend to have the most polling, the average difference is only 3.4 points.That’s barely any gap at all. It suggests the two measures are highly correlated. Indeed, 90% of the differences in the Senate margins across states can be explained by the differences in the presidential margins. When we incorporate the results from the 2016 presidential election into our measure of presidential margins, this 90% becomes 91%. If that were the case when the actual results come in, the presidential and Senate margins in 2020 would be more highly correlated than in any Senate cycle since at least 1980. The bottom line is Senate candidates in each state look to be more tied to the hip to their party’s nominee for president then since at least 1980. There is certainly some ability for Senate candidates to differentiate themselves from the top of the ticket, but not anywhere near as much as there used to be. This could help to explain at least partially why few Republican senators are opposing Trump’s move to put a new Supreme Court justice on the bench before the election.There’s just not much to be gained by going against the President.