Midterm massacre? Or Trumpian exceptionalism?

Woodrow Wilson lost 60, Warren Harding 77. Harry Truman lost 54, Lyndon Johnson 47. Ronald Reagan lost 26, Bill Clinton 54. And Barack Obama? He lost 63.

Incumbent presidents almost always watch their party lose substantial numbers of House seats in their first midterm congressional elections. The exceptions are rare — about as rare as Franklin Delano Roosevelt (gained nine in 1934) and George W. Bush (gained eight in 2002) being in the same sentence.

So the question that lingers in every political discussion this year is a perplexing one: Will a president whose campaign broke every rule, and whose presidency breaks every rule, be able to break with tradition and hold the House and Senate, legislative chambers where he does not even command the full confidence and support of his own majority party?

On the answer to that question rest the destiny of the president’s wall along the Mexican border, the Trump infrastructure initiative, the status of Obamacare, perhaps the shape of the Supreme Court, almost surely the remainder of Mr. Trump’s term.

Seldom — perhaps not since 1994, when Mr. Clinton watched his party relinquish control of the House for the first time in four decades, perhaps since 1938, when FDR was repudiated for his plan to pack the Supreme Court and as a result lost his liberal majority in the House chamber — has so much ridden on midterm congressional elections. And seldom — almost certainly since 1938, when Roosevelt tried to purge his own party of conservatives — has the political world been so complex, contradictory and contrary.

This time, as in 1938 — when Southern conservatives asserted their strength in the Democratic coalition and when prospects for far-reaching social legislation dimmed — the fate of the president, and of the broader political alignment in Washington, depends on many moving parts. It depends, too, on turnout, which almost certainly will be low, around 35 or 40 percent and perhaps lower among Republicans if custom prevails and the party out of power is more energized than the party in the White House.

“The question is not whether people will vote Republican or Democratic,” says Andrew Smith, director of the University of New Hampshire Survey Center. “The question is whether they will vote at all.”

Here are some of the constituencies that may play an outsize role:

• Women. Supporters of Hillary Clinton remain angry about the election of Mr. Trump and are likely to flock to the polls in greater numbers than they did in 2010 and 2014, when Barack Obama was in the White House. Democrats will target middle-age and older women. The model: the Virginia gubernatorial race last year, in which 300,000 “extra” voters turned up at the polls, many of them women from suburban areas.

“The president’s approval ratings among women is much much lower than it is among men,” says Scott Jennings, a onetime aide to Senate Majority Leader Mitch McConnell of Kentucky and a political strategist in the George W. Bush administration. “If the Democrats are reasonably competent about math, they will see that they should try to turn out as many women as possible.”

• Younger voters. Democrats will focus on this group as well, though perhaps more as an investment toward the 2020 presidential election than toward November’s midterms; these voters are notoriously uninterested in voting in non-presidential years. Many of the younger Democratic voters were supporters of Sen. Bernie Sanders of Vermont and have no natural repository for their efforts in midterms. The Democrats’ best hope may be in finding candidates who hug the extremes of consensus politics, but the victory of centrist Conor Lamb in the special election last month in southwestern Pennsylvania suggests that the Democrats’ political pros will lean away from the fringes.

• Supporters of the Tea Party coalition, particularly gun owners. These voters may form a backlash against the anti-gun rallies that so inspire younger voters (above). They are playing defense now, which may end up mobilizing them. A danger for Republicans, however, is that fiscal conservatives congenial to the Tea Party oppose the recent budget deal and may feel alienated. The GOP hope: These voters will get over their anger and decide it’s better to have a Republican in Congress than a Democrat.

• Hispanics. Mr. Trump’s aggressive opposition to immigration, his recent remarks about the Dreamers and his devotion to the Mexican wall may make many voters in this diverse group vulnerable to Democratic entreaties.

• Rural voters. The farther a voter lived from a metropolitan area, the more likely he or she was to vote for Mr. Trump two years ago. Rural voters have leaned Republican for some time — Nebraska, for example, has delivered a total of six electoral votes to a Democratic presidential candidate in the last 80 years, five of them coming in the Lyndon Johnson landslide of 1964 — so this is fertile GOP territory for Republicans.

“Anger and fear drives the votes here as well as ideals, but we always have a drop-off in midterm elections in Iowa,” says Dennis J. Goldford, a political scientist at Drake University in Des Moines. “A lot of Iowa Obama voters switched to Trump. If Trump comes here and says that he needs certain Republicans in Congress, that could help them. And while everyone says the passion is on the Democratic side, Trump is going to work very hard to keep the passion going on the Republican side.”

• Conservatives. This is a surprisingly tricky voter group. They’re not partial to Democrats but view Mr. Trump with suspicion. Still, one of the most visible anti-Trump conservatives, William Kristol, recently wrote that he checked a box declaring “I am a Republican.” Thomas Donahue, president of the United States Chamber of Commerce, has broken with Mr. Trump on some issues, but recently wrote that “sky-high optimism among business leaders is a major indicator that our country and our economy are moving in the right direction.” Count his special-interest group in the Republican column.

Then again, there remain many uncertainties. The conservative Ann Coulter recently said that while Mr. Trump was “a coarse vulgarian;” she was drawn to him in 2016 but is alienated now because “he’s totally walked away from his central campaign promises.” It’s unlikely that voters of the Coulter outlook will vote for a Democrat, however. Maybe they’ll stay home and watch “Roseanne.” It is running, after all, in what television executives call “a revival season.”

David Shribman, a Pulitzer Prize winner in journalism, is executive editor of the Pittsburgh Post-Gazette.