Why Clinton’s margin of victory matters

Less than four weeks remain before one of the most consequential elections in recent American history. No, not the presidential election. Donald Trump has already lost that one several times over — through his serial attacks on women and minorities, the appearance of a videotape in which he describes himself as a sexual abuser, and his overall failure to turn himself into a credible president-in-waiting.

Still at stake on election day, though, is what kind of presidency Clinton will be allowed to have — and that depends mostly on who is elected to the Senate and House of Representatives.

If Democrats win a majority in both chambers, Clinton will be able to pass significant parts of her platform, much as Obama did in 2009 and 2010.

But if Republicans keep their majorities, she’ll have to negotiate with an angry opposition in which the loudest voices are likely to revive the obstructionism they have perfected over the last six years.

And in the likeliest case — a Congress between those extremes with a closely divided Senate and a slim GOP majority in the House — the consequences are unpredictable.

Democratic and Republican strategists broadly agree on the range of possible electoral outcomes.

The Senate is balanced on a knife edge. Republicans hold 54 of the Senate’s 100 seats; if Democrats net four additional seats, they’ll have 50, and can rely on Vice President Tim Kaine to break the tie. The closest races are in Pennsylvania and New Hampshire, where Republican incumbents are struggling to hold off Democratic challengers. But Republican senators could also lose their seats in North Carolina and Missouri.

“We need to win three of those four seats to get to 51,” GOP pollster David Winston told me.

He said he was worried by a recent poll — taken immediately after the Trump videotape surfaced — that suggested only 74% of Republican voters were still supporting the nominee.

“If he comes in with 74% of the Republican vote, the party is completely blown out of this election,” Winston said. “He needs 90%.”

The House is more complicated — and much harder for Democrats to take. Republicans currently hold 246 of 435 seats; Democrats need a net gain of at least 30 to win a majority.

“It’s possible to imagine, but it’s unlikely,” Democratic pollster Mark Mellman said. He estimated that Clinton would need to win the presidency by a margin of 10% or more for the House to change hands. That’s not likely to happen; the last candidate to win by more than 10% was Ronald Reagan in 1984. (In the most recent polling released by NBC News and the Wall Street Journal, Clinton was leading by 7%.)

Why is it so difficult to flip the House? Redistricting, which guarantees that most incumbents can rely on loyal partisans to keep them in office. In the 2012 election, Republicans lost the popular vote for all House candidates by more than 1.4 million, but still won a majority of seats.

So President Clinton will almost certainly be negotiating with a Republican House — one, moreover, whose majority will be splintered into angry factions, pro-Trump and anti-Trump.

If her electoral majority is huge, she’ll be able to claim some measure of a popular mandate — even though some Republicans will argue that she won only because they nominated a bad candidate (or, for die-hard Trumpists, because the election was “rigged”).

That would give her an opportunity to seek cooperation from pragmatic Republicans — yes, there still are some.

“If there’s a backlash against the awful nature of the campaign, there could be real public pressure to let her get a few things done,” Norman J. Ornstein, a scholar of Congress at the American Enterprise Institute, told me. “She could pass an infrastructure program; that’s got support in both parties. She could pass some technical fixes for Obamacare.”

But that’s about all — and that’s “a rosy scenario,” he added.

In a darker scenario, “Republicans will decide that they want to replay what they did in 2010 and 2014 — delegitimize the president and block whatever they can,” Ornstein said.

Clinton can help nudge the new Congress toward the first scenario if she continues to proclaim her desire to be a president for members of both parties, a theme she’s taken up over the last few weeks of the campaign. Even more important, she could name a Republican or two to her Cabinet and quickly launch bipartisan conversations to search for common ground.

That won’t transform a GOP that will still be dominated by her fiercest critics — but it will improve her chances of accomplishing the changes she wants.

First, though, she has to win as many votes as she can. The presidential margin will matter as well as voters’ choices for members of the Senate and House.

No matter what Trump does over the next four weeks, no matter what new revelations about the candidates emerge, and no matter what the polls say, every vote will still count.

Still at stake is what kind of presidency Clinton will be allowed to have.