If Joe Biden prevails at the polls next month, the very thing that made him so formidable in his contest against Donald Trump this year is going to make governing extremely precarious for him.

I'm talking about Biden's facility at straddling the Democratic Party's very wide ideological divides — from Bernie Sanders to Michael Bloomberg on economic policy, and from highly educated urban progressives to Midwestern working-class voters who are conservative on culture-war issues. Biden falls somewhere in the middle on both economics and culture, but his campaign has been deliberately vague about exactly where. That's because he and his team understand that his electoral strength against a highly divisive opponent comes from him appearing to be as close as possible to an imaginary generic Democrat.

But as we've started to see with rising calls for Biden to take a stand on whether, after an imagined landslide, Democrats should pack the Supreme Court with new justices in retaliation for unprecedented hardball tactics on the part of Republicans in the Senate over the past several years, partisan gales in our politics are fierce. The left wing of the party is clamoring for court packing. Biden, a temperamental moderate and institutionalist who wants to strive for conciliation with Republicans after the election, is almost certainly against it. But it's not in his interest to inflame the left prior to the election by making his opposition explicit. So he's trying to hover above the partisan fray. But that will be impossible when it comes time to govern — and yet the threat from Biden's left flank will remain, as will the danger posed by Republicans who have ample incentives to ensure that the new president's overtures to the center-right come to nothing.

That is the bind a newly minted Biden administration is going to find itself in.

Biden will want to try governing as if he's leading a national unity coalition, picking early centrist priorities (like combating COVID-19 more aggressively and competently than Trump and passing economic stimulus in the form of a large infrastructure bill) that some Republicans in Congress might be willing to support, in the hope that on the basis of increasing comity in Washington, bigger progressive priorities will be achievable down the road.

But two ideologically opposed factions in our politics will seek to scuttle this approach to governance. First, the left (led by Rep. Alexandria Ocasio-Cortez and other members of "The Squad" in the House) will demand that its ambitious agenda (including the Green New Deal and single-payer health care) take center stage and strongly oppose the effort to sideline it in favor of proposals Republicans might support. At the same time, and from the opposite ideological direction, House Republicans will work with Republicans in the Senate who have presidential ambitions (including Tom Cotton, Josh Hawley, Ted Cruz, Rand Paul, and Marco Rubio), and those fearing primary challenges from the right in 2022, to keep members of the party unified in opposition to the new president's outreach efforts.

We've seen this dynamic throughout the current election year. Trump didn't want to be running against Biden (or the center-left Pete Buttigieg). He wanted his opponent to be Bernie Sanders, Elizabeth Warren, or one of the other dozen or so candidates who positioned themselves further to the left during the Democratic primaries. This has been obvious from the way the Trump campaign has continually tried (and failed) to portray Biden, despite his middle-of-the-road record, as a tool of the left. It's also obvious from the way vice president Mike Pence, during his debate last week with Biden running-mate Kamala Harris, repeatedly pointed to left-leaning positions Harris staked out in her own failed run for her party's nomination.

Republicans believe their party benefits from facing a Democratic Party that can be convincingly portrayed as a vehicle for left-wing extremism. Which means that they would much rather run against a Democratic Party dominated by Ocasio-Cortez than Joe Biden. And of course Ocasio-Cortez and her ideological allies would much rather drive the Democratic agenda than cede it to a Biden White House aiming to find allies on the other side of the aisle.

From a Biden inauguration onwards, Republicans will be working to build a GOP wave during the 2022 midterm elections that repeats the ones the party enjoyed in the first midterms during the last two Democratic administrations. Two years into Bill Clinton's presidency, in 1994, Republicans took control of the House for the first time in 40 years, in the wake of the administration's failed push to enact a program of universal health care. Sixteen years later, two years into Barack Obama's presidency, the GOP used hostility to the passage of the Affordable Care Act to fuel a wave in which Republicans made a net gain of 63 seats in the House, the biggest shift since 1948.

Something similar will only be possible in two years if Biden's efforts at conciliation fail.

The leftward factions of the Democratic Party would likewise prefer to be able to say that Biden's efforts at outreach to Republicans failed — and to have a virulently right-wing Republican majority in Congress to run against in 2024.

All of these incentives will make Biden vulnerable. If he defeats Trump in a historic landslide, that will grant him a honeymoon period during which he will be able to take his best shot at breaking the cycle of polarization and governing from something approaching the bipartisan ideological center of the country. But that period will be brief.

If the effort fails, Biden will be forced to fight the same centrifugal forces pulling all of us toward the extremes. Whether and how long he can resist them is likely to be the most important question hanging over his presidency.