Norman J. Ornstein is a resident scholar at the American Enterprise Institute.

How does a divided United States, gripped with a combination of angry populism and partisan tribalism, govern with a president who lost the popular vote but a Congress that is primed to implement swaths of his agenda — and expects the president to bow to some of its policy preferences along the way? How can this dynamic play out in a way that respects the fact that Republicans control the executive and legislative branches but more than half the country does not embrace a sharply ideological or radical agenda? How do we keep extreme policies at bay?

Donald Trump’s choice of alt-right leader Stephen K. Bannon as top White House strategist suggests that the worst instincts of the president-elect remain intact. The hopes for governance hewing closer to the center, and respecting all those living in the United States, rest with the Senate, and with the behavior and outlook of senators in both parties.

For Senate Republicans, this is a critical moment to heed a call their colleague Lindsey Graham (S.C.) made earlier in the campaign — there is a time to put country ahead of party. Reflexive party loyalty now, including blindly confirming all of Trump’s executive and judicial nominees, would violate that maxim. Republicans need to vet thoroughly and firmly all nominees, but especially those who violate basic standards of moderation, civility and respect for constitutional norms. That means racially divisive and bombastic figures such as Milwaukee County Sheriff David A. Clarke, who stunningly called on his allies during the campaign to ready their “pitchforks and torches,” and who has been rumored to be Trump’s top choice for homeland security secretary, should be rejected.

Should the equally divisive Rudy Giuliani be nominated for a position in Trump’s Cabinet, he should be subject by Republicans as well as Democrats to probing confirmation hearings, including his highly questionable relationship during the campaign with the New York office of the FBI, the over-the-top accusations he directed at Hillary Clinton and his enrichment from a range of autocratic regimes. And whatever figures are eventually confirmed, it is vital that Senate Republicans provide oversight with vigor and integrity to make sure that offices are not abused. Similarly, vigorous oversight is needed to keep the country from becoming a kleptocracy, where policies that serve the holdings of officeholders, including the president, prevail over the needs of the country.

Senate Republicans should also make sure that destructive policies, whether to increase the debt with unpaid-for tax cuts, blow up agencies, make mindless cuts in regulations, pursue trade wars, accept torture, move us closer to an all-out war with Islam or curtail civil liberties, do not get jammed through Congress. They cannot succumb to the pressure sure to come from the radical right to kill the filibuster not just for nominations but also for legislative actions. Not every Senate Republican will heed this call. But from a group including Graham, Jeff Flake (Ariz.), Susan Collins (Maine), Lisa Murkowski (Alaska), Shelley Moore Capito (W.Va.), Lamar Alexander (Tenn.), Bob Corker (Tenn.), John McCain (Ariz.), Johnny Isakson (Ga.) and Ben Sasse (Neb.), among others, there should be sufficient numbers to provide a check and balance against authoritarian and divisive rule.

What about Senate Democrats? They will be tempted to adopt the Republican playbook from 2009, when Democrats controlled Washington: Vote in unison against everything, filibuster everything, even those things you like, to obstruct action and make it look ugly, allow damage to the country in the short term to reap political rewards in the next election. That would be a mistake.

Democrats have a chance to use their filibuster power not just to obstruct but also to improve. The first opportunity to do that will likely be on an infrastructure bill. Financing a good portion of the package through repatriated corporate profits invested in long-term, low-interest infrastructure bonds is one key. Making sure that a significant share of the program goes to green energy, broadband, cybersecurity and the electrical grid should be strong demands. So should a guarantee that any corporate tax reform that is a part of the deal not be simply another giant tax cut and giveaway.

The second chance may come on health policy. Senate Majority Leader Mitch McConnell (Ky.) has pledged to repeal Obamacare as an early priority — now contradicted by the president-elect. Democrats can bargain for a revision of Obamacare — call it Trumpcare — that keeps exchanges and the individual mandate to preserve a meaningful protection for preexisting conditions and prevents a new lifetime coverage limit, and also strengthens risk corridors to prevent huge premium fluctuations. In return, they can drop the employer mandate, strengthen markets and reform malpractice insurance.

Filibustering an extreme Supreme Court nominee, and blocking legislation to privatize Medicare or Social Security, to eliminate or eviscerate the Environmental Protection Agency or the Consumer Financial Protection Bureau or to give more giant tax cuts to the super-rich — all these make sense for Democrats and for the country. But embracing the McConnell tactic of mindless obstruction does not. Right now, our hopes of having the better angels in our system prevail over our worst forces and instincts rest with the Senate. Let’s hope its members respond.