Democrats have seen their odds of a massive blue wave return over the past month to about where they were at the end of 2017, when President Trump was about as popular as drinking alone in Minnesota. But as with their polling nosedive earlier this year, which caused so much anguish on the left, it's not at all clear what is causing this uptick in the party's fortunes.

No matter where you look, the numbers are better for Democrats than they were two months ago. The blue wave faces its most challenging journey through the Senate, where Democrats must flip at least two Republican-held seats and protect vulnerable incumbents in Trump landslide states like North Dakota, Indiana, Missouri, and West Virginia. Yet if recent polling is to be believed, that might be exactly what would happen if the election were held today. Democrats Kyrsten Sinema (Arizona) and Jacky Rosen (Nevada) have led every public poll of their races in seats now held by Republicans.

Another poll had Democrat Phil Bredesen, a popular former governor, leading Rep. Marsha Blackburn (R) by 6 points for the open Tennessee seat vacated by Sen. Bob Corker (R). Corker, like retiring Arizona Sen. Jeff Flake (R), made the courageous decision to flee from Trumpism rather than to confront it in a primary, opening up winnable seats for Democrats. If Democrats win all three of those contests, they can afford to lose one of the races in deep-red territory. While Indiana and North Dakota haven't been surveyed recently, an early July poll had incumbent Sen. Claire McCaskill (D) trailing her Republican opponent, Josh Hawley, by 2 in Missouri.

Elsewhere on the Senate map, things are looking even worse for the GOP. One Republican pollster had conservative Democratic Sen. Joe Manchin up 10 points over his challenger Patrick Morrisey in West Virginia, a state Trump won by 41 points. That's in line with the last three polls of the race, all of which have Manchin leading big. And in Florida, incumbent Sen. Bill Nelson (D) leads Republican Gov. Rick Scott, who has already burned through millions. Democratic Senate incumbents in states more narrowly captured by Trump in 2016, like Michigan, Wisconsin, and Pennsylvania, appear out of reach for a GOP that had once hoped to expand its Senate majority by way of a friendly map and a strong economy this year.

The standard caveats apply for all of these Senate races — this is relatively sparse polling, in the middle of the summer, and most of it has yet to be confirmed by multiple pollsters. But the denser, more frequent generic ballot polling has also returned to the kind of substantial lead Democrats need to overcome the GOP's post-2010 gerrymandering and take back the House of Representatives.

As of today, Democrats have to be considered at least narrow favorites to win the House, possibly by a comfortable margin. That isn't just an extrapolation of the generic polling but also based on close observations of individual House races by groups like Sabato's Crystal Ball and the Cook Political Report, both of which recently adjusted their forecasts for a number of seats. And recent gubernatorial polling has looked bad for the GOP too, with Democrats closing in on Republican-held seats in states like Michigan, Tennessee, Georgia, Maine, and Florida in addition to expected flips in Illinois and New Mexico. That will soften the blow as Democrats seem condemned to watch popular Republican governors re-elected in deep-blue Maryland and Massachusetts.

What's going on here? Just a couple of months ago, observers were declaring the Democrats' pursuit of the Senate and the House dead in the water or at least far from assured. The swings in the polling average are difficult to pin to particular events. In 2013, for instance, Republicans reversed a Democratic advantage on the generic ballot after the Obama administration's botched rollout of its health-care website on Oct. 1. By the end of that month, a 5-point Democratic advantage had been turned into a 2-point Republican lead. The GOP would ultimately win the national House vote the following year by almost 6 points. Cause and effect.

But the narrowing of the polls in favor of Republicans between January and June of this year made little sense. The GOP's tax cut — the only significant piece of national legislation the party has managed to pass despite total control of Congress and the presidency — was radioactive when it passed and only slightly less unpopular today. As Americans discover that corporations have no intention of sharing their tax windfalls with workers by raising wages, the law is likely to fall even further in public esteem.

The improvement in Democratic fortunes likewise has no clear policy cause. The New York Times published its blockbuster story about the White House's cruel policy of family separations at the U.S.-Mexico border on April 20, but the GOP continued to narrow its deficit in polling through the end of May. Most interestingly, the president's approval ratings have barely budged at all this summer, even through the tempest of the family separation scandal, the escalating tariff war, and Trump's bizarre, toadyish embrace of Russian dictator Vladimir Putin in Helsinki.

One possibility that might not be terribly satisfying for polling junkies is that these numbers have never really moved very much at all. Democrats led by 8 points on the first post-inauguration poll of the generic congressional ballot, and that's close to what their average advantage is now. The swing to the GOP earlier this year may simply have represented a short-term disengagement from politics from the Democratic base, which is becoming activated again as Election Day draws near.

The meta-story here is actually very simple: An unpopular, unstable president has pursued unpopular, unnecessary policies in tandem with an unpopular, craven congressional majority, resulting in a consistent polling deficit for the party in power. If President Trump and his allies continue on this disastrous course through Election Day, which they show every sign of doing, they will almost certainly be punished by the voters. The only question is the scale of the retribution and whether it will be enough to give Democrats control of one or both houses of Congress. None of this is even particularly surprising at a historical level, as the president's party almost always loses seats in the midterm elections. This Republican Party wouldn't even be the first to manage the seemingly impossible feat of bleeding out seats as the economy booms.

But to its pursuit of toxic policies and its difficulty fighting against the inevitable partisan headwinds, the GOP has added an additional handicap: the president himself. Donald Trump is an extraordinarily tiresome person whose constant mewling, bottomless self-pitying, grandiose conspiracy-mongering, and unpresidential grandstanding repulse a solid and consistent majority of people who encounter it. America is a country that in many ways yearns for a return to normalcy, and is probably willing to let it happen even under Trump's misrule. But the president himself simply won't allow it.

President Trump has also deliberately forsaken every single opportunity he has been given to speak or govern in ways that might mollify his opponents or build a broader coalition. Worse, he has spent much of this year on a self-destructive quest to light the embers of a global trade conflagration, which now threatens the health of the economy and the party's fortunes in key swing districts.

With his every raving Twitterance, he redoubles the determination of millions of people to crawl over fields of broken glass to vote against the GOP. What must be most galling for Republican leaders is that with unemployment at 4 percent, they could probably hold both chambers of Congress if their demented field marshal could just button up his pie-hole for three minutes at a time about Russia and the Mueller investigation, or if they could convince him to occasionally go on TV and act even remotely presidential instead of jetting off to another one of his meandering, hate-filled rallies.

Worst of all for Republicans, if there is an explanation for polling swings, it's this: Democrats have been at their strongest over the past two years when they have come together to fight the Trump administration with fiery unity. That was true during the health-care fight in the summer of 2017, and it was true during the tax-cut battle that December. When that unity has dissipated, or when Democrats in Congress have lost their fighting spirit, they have watched their polling numbers decline. Senate Majority Leader Mitch McConnell is pondering a plan to jam Supreme Court nominee Brett Kavanaugh through the Senate on the eve of the midterms, believing his inevitable confirmation will demoralize the left. But he has it exactly backwards. As long as Minority Leader Chuck Schumer can keep most of his vulnerable caucus members in united opposition to Kavanaugh, his confirmation will give Democrats and their voters a fresh reason to turn out and deprive Republicans of their congressional majorities.

As of today, it seems as likely as not that these dynamics may deliver the House and the Senate to the Democrats.