Political events in 2017 offer many reasons to believe that 2018 will bring a sweeping rebuke to Donald Trump at the ballot box. With the mandatory caveat that nothing should be taken for granted or regarded as predetermined, the past year has provided all the necessary preconditions for a backlash which—depending on its size—threatens to undermine or possibly eliminate the advantages Republicans have used to hold power without the benefit of a natural electoral majority.

Just as waves originate in the open sea before crashing ashore, the requirements for a political upheaval are in place months before becoming visible. This means it isn’t premature to look at present conditions for guidance on where we may be heading next fall. Political waves are a reaction to a deeply felt situation that disproportionately motivates one portion of the electorate while everyone else exhibits normal or depressed levels of engagement. This differential tilts the playing field toward candidates aligned with the energized group and overrides factors that determine the outcome of normal elections like candidate strength, campaign effectiveness, local issues, and the natural partisan leaning of the electorate. This is why wave elections will generate unusual results, producing competitive elections in atypical places and taking down candidates who would survive ordinary years.

Early signs of a brewing wave will appear in fundraising figures, candidate recruitment, measures of interest in the election, and—imperfectly—in special and off-year election results and presidential approval ratings. So where do things stand? Last October, Politico reported that “historic” fundraising totals among a record number of Democratic House candidates were “alarming GOP strategists”:

Animated by opposition to President Donald Trump and the Republican congressional majorities, at least 162 Democratic candidates in 82 GOP-held districts have raised over $100,000 so far this year, according to a POLITICO analysis of the latest FEC data. That’s about four times as many candidates as House Democrats had at this point before the 2016 or 2014 elections, and it’s more than twice as many as Republicans had running at this point eight years ago, on the eve of capturing the House in the 2010 wave election. Nearly three dozen Republican incumbents were outraised by Democratic challengers in the third quarter of [2017]—a stunning figure. Nine GOP incumbents already trail a Democratic opponent in cash on hand, increasing the likelihood that many veteran incumbents will face tough opposition for the first time in years.

Leading the way are an unprecedented number of women who have been inspired to run for office and many first-timers who felt they had to get off the sidelines and fight back against the Trump administration, backed by a spike in small-dollar contributions reflecting intense grassroots energy. This surge in candidates and fundraising reflects disproportionate interest in the 2018 election by Democrats, which in turn is reflected in lopsided figures in the generic congressional ballot, a measure of how voters would cast their congressional ballots if the election were held today. A number of pollsters gather data on the generic ballot, and an average of these surveys gives Democrats a historically large double-digit advantage—an advantage with predictive value, even this far away from Election Day.

Then there are the 2017 election results. Republicans won five special house elections last year, but with margins between seven and sixteen points below their 2016 totals. These elections took place in what should have been the safest of districts in Georgia, South Carolina, Kansas, Montana and Utah, and were harbingers of the drubbing Republicans suffered on Election Day in Virginia, New Jersey and smaller jurisdictions across the country, followed by Roy Moore’s loss in Alabama last month. These victories were powered by disproportional turnout favoring Democrats of the kind we would expect to see during an electoral wave, and—more disconcerting to Republicans should it continue—by the widespread abandonment of suburban voters who were once a cornerstone of GOP strength.

Finally, there is Donald Trump, the single greatest force stirring the political ocean. Presidential approval is correlated with off-year results, with the out-party’s natural advantage enhanced when the incumbent’s performance is poorly regarded. Since he took the presidential oath, Trump has realized historically low approval scores and alienated a majority of the country as he has governed for his core supporters and the Republican donor class. A normal administration would look at this situation and decide that a significant correction is necessary to avoid a mid-term disaster. But anyone who expects Donald Trump to change isn’t paying attention.

So we begin 2018 with the necessary conditions for a wave in place. But November is a long time from now and the political system is under great stress. The intervening months are unlikely to change the dynamics propelling Democrats toward an off-year rebound—but they may well require vigilance to defend the integrity of the political system from attacks by a president with anti-democratic impulses and a party that enables him out of the fear of losing power.