The last two weeks have seen some significant movement in Democrats favor. First, there were the impressive results from last Tuesday's elections. This week, we’ve seen two polls — one by Quinnipiac and one by Marist — that show Democrats with a congressional ballot advantage of +13 to +15. Three other recent polls — ABC/Washington Post, Fox, and NBC/Wall Street Journal — show Democrats with an advantage of anywhere from +7 to +15.

These are political wave numbers.

Quinnipiac and Marist Generic Ballot Polling

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But, is it realistic to think that Democrats could retain this kind of advantage into 2018? Moreover, given the Republican structural advantage — gerrymandered seats plus Democrats’ self-sorting into urban areas — is that kind of margin even big enough to net 24 seats?

My colleague David Wasserman has been digging into the question of just how big of a wave Democrats need to get in order to surf into the majority. The short answer: they need to see a generic ballot advantage of +8 or more, which roughly translates to getting at least 54 percent or more of the national House vote in 2018.

The last time Democrats enjoyed a margin of +8 or more in a mid-term year was 2006. That year, Democrats won the House vote by 8.5 percent. The last time that Democrats got into the double digits was 2008 when they carried the House vote by D+11. This has led to a lots of talk that Democrats can only hit significant margins of victory in presidential elections when their base is more engaged and involved. It also helped to have a transformational candidate - Barack Obama - at the top of the ticket. Something they obviously don't have in 2018. But, there is precedent for Democrats winning the House vote by double digits in mid-term years. In the post-Watergate midterm of 1974, Democrats won by a whopping 17 points. In Ronald Reagan’s first midterm of 1982, Democrats won the House vote by 12 points.

Democratic House Vote Margin in Midterm Elections

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Of course our country is more polarized than it was in the 20th Century. And, redistricting/gerrymandering has gotten more sophisticated. As everyone knows by heart right now, just 23 Republicans sit in seats carried by Hillary Clinton.

I don't dismiss those realities. However, I am having a nagging sense of deja vu - a feeling like I've been here and heard these same arguments before. Way back in 2006, my boss Charlie Cook was warning that the year was shaping up to be a wave year. I argued that unlike the last wave election of 1994, the party holding the White House was much better prepared. Republicans in 2006 had significant financial advantages. They had structural advantages. And, Democrats couldn’t sneak up on Republicans as Republicans had to Democrats in 1994. Obviously, my theory was wrong and Charlie was right.

Lots has changed over the course of those years — the rise of SuperPACs, digital targeting, data analytics. But, there has been one important, overarching constant: a candidate can control for many things, but he/she can’t change the political mood. If it is with you, you get an extra advantage you may or may not deserve. When it is against you, even the best, most prepared candidates can lose. This was true in 1994. It was true in 2006. It was true in 2010.

Getting a tax bill across the finish line isn’t going to be enough to change the mood of the country. It is going to take something much more significant to do that. A good economy is helpful to the GOP as it can cut down on some of the headwinds coming at them right now. But, it’s not clear to me that it’s enough to fundamentally alter the way voters see Congress, the GOP and the President.

In 2016 we made the mistake of rationalizing away the prospect of a Trump victory. He was too unorthodox. He couldn't possibly sustain momentum through the grueling primary campaign. We should not make same mistake in 2018. Sure, a lot can change between now and next November. And, Democrats have a narrow path to 24 seats - even with a big wave or tailwind. But, do not ignore what’s right in front of us. A wave is building. If I were a Republican running for Congress, I’d be taking that more seriously than ever.