(CNN) Republicans dominated the midterm elections in 2010 and 2014 in part because voters who supported Barack Obama in presidential years voted for Republicans or stayed home when he wasn't on the ballot.

The turnout advantage Republicans enjoyed during the Obama years may no longer exist as we head into 2018, and in fact, there are some signs that Democrats may be the one with a decisive turnout advantage this cycle.

This shift in reality may not be reflected in much of the high-quality polling so far this year because much of it has been done among registered voters instead of the more refined category of likely voters. In other words, current polling may not fully reflect Democratic strength heading into the midterms.

This model I feel is one that captures both the fact that past voting is usually highly predictive of future voting and that there may be some new voters casting ballots this year.

In five of the seven races polled, the Democratic candidate either did better or no worse when Monmouth switched from registered voters to likely voters. And in the two races where the Republican candidate does better with the switch to likely voters, the change was 2 points or less.

The average shift has been 2 points in favor of the Democratic candidate.

A 2-point shift may not seem like a lot but it could be huge if it holds across the board. Democrats right now hold roughly a 7-percentage point edge on the generic congressional ballot. That's right in the area where they need it to be to have a net gain of 23 seats to take control.

Per a vote-seat curve I created based off the historical relationship between votes and seats won, each point gained nationally is worth somewhere in the neighborhood of a net pickup of three to four seats.

In a hypothetical scenario, 2 points could mean the difference from a net gain of 21 seats for the Democrats and a Republican majority to a net gain of 27 seats for the Democrats and a Democratic majority.

Now the races Monmouth polled may not end up not being representative of the nation at large. The state of play can also shift before the midterm.

But if what these races are telling us holds, 2018 is about to be very unusual and certainly very different than what occurred in the last few midterm elections.

In 2014, Republicans did about 6 points better in the final generic congressional ballot among likely than among all registered voters. That turnout gap (between registered and likely voters) was about what we saw in the 2010 election as well, according to a study by FiveThirtyEight's Nate Silver . The turnout gap in 2010 may have been responsible for Republicans gaining their House majority that year.

Indeed, when I looked at the turnout in every midterm election since 1978 earlier this year , I didn't find a single cycle in which Democrats held a turnout advantage over Republicans.

The fact that Monmouth seems to be showing something different doesn't mean it's wrong. In fact, it makes a lot of sense. We know that Democrats have been showing up in large numbers in elections during this cycle so far. This includes special elections, gubernatorial elections and in primaries

We also know that Democratic turnout relative to Republicans tends to be highest in midterms when there is a Republican president