The results of the special House election in Montana last week—where the Republican candidate won by a single-digit margin in a "deep red" constituency—are best interpreted as providing one more data point in favor of the conclusion that the national political environment has undergone a substantial, though not necessarily fatal, shift away from the Republicans and towards the Democrats since the inauguration of Donald Trump. This swing is thoroughly consistent with the larger back-and-forth pattern of partisan competition for the past 25 years, in which achieving unified control of Congress and the presidency has consistently rendered a party vulnerable to immediate popular backlash.
On the basis of history alone, then, there is good reason to expect the 2018 election to produce a more favorable outcome for the Democrats than 2016. How much more favorable, however, is impossible to foresee so far in advance of the vote. Eighteen months is a long time in politics under normal conditions—conventional wisdom would have bet overwhelmingly at this stage against the final results of the 1992, 1994, 1996, 1998, 2006, 2010, and 2016 elections. And we are not currently in normal conditions; if anything, the speed of political developments is unusually quick these days, and the level of uncertainty about future events is even higher than usual.
From today's perspective, there are at least two major political issues that are likely to exert a significant effect on the results of the 2018 midterms. One is the Trump-Russia connection and its associated complications, stretching from Wikileaks to Michael Flynn to Jared Kushner to the FBI. Though the appointment of a special prosecutor suggests that this story isn't fading anytime soon, it's impossible to predict what directions it will go over the next year and a half, and what response it will produce in the mass electorate. The key question lurking behind the Russia affair—the extent of Trump's personal knowledge of, or involvement in, any illegal or shady activity—is not much clearer now than it was at the beginning of his presidency, and there is no guarantee that the Mueller investigation will provide a clear answer before November 2018.
The second significant and unpredictable factor in 2018 will be health care: specifically, whether the Republican-controlled Congress will succeed in enacting health care reform, and—if so—what form such legislation will take. The House narrowly passed a reform bill in early May, but Senate Republicans have opted to write their own version from scratch—an effort that already seems to have encountered serious problems. Whether the Senate will be able to pass a health care bill of its own is itself difficult to determine (Republicans can only afford two defections out of a 52-member conference). And whether any bill crafted to survive the Senate gauntlet could then serve as the basis for successful reconciliation negotiations between the two chambers is itself very difficult to know in advance, at least until its major provisions come into greater focus. Whatever the Republicans wind up doing about health care, from passing nothing at all to pushing through a substantial rollback of the Affordable Care Act, is almost certain to carry an electoral cost in 2018, but how big a problem health care will be for the party is impossible to ascertain before it's apparent what the policy change (if any) will be and when it will take effect.
So even excluding the unforeseeable events—foreign crises, terrorist attacks, economic trends—that may well occur and further influence the electoral balance between the parties, we have little reason to believe that the political climate will remain stable between now and November 2018. It's more likely that things get better for the Democrats than for the Republicans, given the performance of the Trump administration and the 2017–2018 Congress so far, but the range of plausible outcomes is exceptionally wide. Due to a structural Republican advantage in the ways that House districts are drawn and in the specific Senate seats up for election in 2018, even a decided nationwide pro-Democratic trend may not prove sufficiently strong to hand the party control of either chamber.
The special election approaching on June 20th in what has become a nationally representative suburban Atlanta House district will almost certainly be treated as an electoral bellwether by the news media. But even if the results accurately reflect the political environment of the moment, we're still so far away from the midterms that there is little sense in interpreting the Georgia race as an indicator of which party is favored to win the most seats in 2018. Throwing up your hands and saying "it's just soon to tell" is not a good strategy for advancing one's career in public punditry, where the constant reading of tea leaves is part of the job description. But in this case, it is just too soon to tell—so beware of anyone who says differently.