For years now, a defining assumption of the 2016 race has been that the House Republican majority is fundamentally not on the ballot. But thanks to Donald Trump, one of the leading House election analysts in America says it may be time to reconsider that assumption.

Folks, it may be time to rethink whether the House could be in play in 2016. https://t.co/NtA8rniKuh @CookPolitical — Dave Wasserman (@Redistrict) March 18, 2016

Dave Wasserman is the US House editor of the Cook Political Report, the gold standard for granular analysis of congressional races. The story he's teasing with this tweet doesn't say Democrats are likely to win a majority in November. It really just says that he's downgrading Republicans' chances in 10 races. But it says that a scenario that was previously thought to be simply impossible is now in the realm of possibility.

Why Trump changes everything

To see why a Democratic majority suddenly seems possible, just read Andrew Prokop's pre-Trump explanation of why it seemed so unlikely:

Democrats currently need to pick up 30 seats to retake the House. The last time an incumbent president's party did that was 1964, when Lyndon B. Johnson won a 23-point victory over Barry Goldwater and Democrats romped nationwide to a 37-seat pickup. An incumbent president's party has not come even close to that since. Richard Nixon and Ronald Reagan each won 49 states in their landslide reelections of 1972 and 1984, but they only picked up 13 and 16 seats in the House, respectively. And landslides like those are all but inconceivable today in our increasingly polarized politics. When presidential elections are closer, the makeup of the House tends not to change very much.

Democrats couldn't win a House majority because the only way to generate a wave election under normal circumstances is as a backlash against an unpopular incumbent president. The exception that proved the rule was 1964, when Republicans nominated an extremely unpopular candidate who deeply divided their party.

That's a crazy thing to do, so most analysts simply assumed it wouldn't happen. And yet it's happening!

Republicans still have big advantages

The high odds of a Trump nomination and the fact that any alternative to a Trump nomination would almost certainly entail some kind of party-crushing convention hijinks mean that a Democratic wave is definitely on the table in a way it wasn't previously.

But there is a big distance between possibly and likely.

Recall that in 2012, not only did Barack Obama win a majority of the popular vote but more people voted for Democratic House candidates than for Republican ones. Nonetheless, Republicans won a majority of House seats due to gerrymandering and favorable geography. These advantages exist, along with the standard incumbent advantage and the reality that Trump would still be running in the context of a much more polarized country.

That polarization could put a floor beneath Trump's vote share, or it could mean that center-right voters who won't back Trump will nonetheless vote to reelect a Republican House to check Hillary Clinton. Paul Ryan will probably be speaker in 2017 just as much as he is today. But while four months ago the idea that he might not be seemed totally crazy, as of mid-March it doesn't seem crazy at all.