Election Day remains about seven months away. Democrats need a net gain of 23 seats to recapture a majority.
But every midterm campaign represents a referendum on the president's performance, and Trump remains broadly unpopular outside the GOP. David Wasserman, a leading analyst of House races at the Cook Political Report, calls Ryan's announcement "a deep blow to his party's morale" and says the odds of a Democratic victory in November have risen to 75 percent.
Ryan is among the approximately 40 House Republican declining to seek re-election — about twice the number of Democrats leaving. His departure will encourage colleagues unsure about their prospects to make the same decision.
Indeed, another House Republican, Dennis Ross of Florida, followed Ryan by announcing Wednesday that he won't seek re-election either. Wasserman notes that 57 other Republicans represent districts in states where the legal deadline for declaring campaign plans has not arrived.
Ryan's stature within the party, burnished by his 2012 vice presidential nomination and career-long commitment to tax cuts, has made him a prodigious fundraiser for other Republicans. The $44 million he raked in as a House leader during 2017 represented a non-election-year record.
Impending retirement reduces his financial drawing power. And because he intends to remain in his post through the end of the year, would-be successors Kevin McCarthy and Steve Scalise won't have the marquee value the speaker's gavel provides.
A Democratic House would have immense policy consequences for the Republican administration. While road-blocking conservative priorities, it would boost chances for bipartisan compromise on issues such as infrastructure.
It would hardly assure that Trump's opponents could force him from office. To begin with, an impeachment drive could fail to attract a majority of the House.
Even if it did, conviction on impeachment charges and removal from office requires a two-thirds vote in the Senate. Democrats currently control just 49 seats in the chamber, with prospects for only a handful more in the most optimistic election scenarios.
But the career decision Ryan announced Wednesday morning amounts to a forecast of political weather for 2018 and beyond. The storms around Trump keep rising.