Let’s start with some simple math. Republicans are expected to hold 52 Senate seats in the 115th Congress. Of those 52 Republicans, 12 — Susan Collins, John McCain, Lindsey Graham, Ben Sasse, Mike Lee, Rob Portman, Pat Toomey, Jeff Flake, Dean Heller, Lisa Murkowski, Cory Gardner, and Dan Sullivan — either refused to endorse Donald Trump or rescinded their endorsement and never clearly restored it during the campaign.

Of the 40 remaining Senate Republicans who did endorse Trump, more than a few have voiced doubts about the president-elect. Rand Paul called him a “delusional narcissist.” Ted Cruz said he was “utterly amoral” and “a pathological liar.” Marco Rubio warned that Trump was a “con artist” who was too “erratic” to control nuclear weapons. Lamar Alexander said Trump was “driving the presidential campaign to a new low.” Though all these Republicans eventually backed Trump out of loyalty to party, no one believes they changed their minds about his basic fitness for office.

Plenty of other congressional Republicans stayed quiet — or as quiet as possible — on Trump. But it breaks no confidences to say that in conversations with both Republican politicians and staffers, the dominant emotions I’ve heard have been despair, fear, and bewilderment. Those emotions are now joined by the unexpected pleasure of victory — but they are not gone.

This, then, is where Trump’s presidency begins: with a closely divided Senate, a supermajority of senators who refused to back his candidacy, and a super-super-majority who harbor grave doubts about his fitness to serve. Assuming Democratic unity, it will only take three Republican defections on any given issue or nomination to create an anti-Trump majority in the chamber. Republicans would be wise to use the narrowness of their majority to curb the incoming president’s worst instincts.

Republicans no longer need to fear Hillary Clinton

During the campaign, Trump was saved by Hillary Clinton — Republicans worried that she'd trigger a Democratic wave that could cost them the House and the Senate. For many top conservatives, the stated reason for endorsing Trump was to stop Clinton.

“There are three words that ought to scare everyone in this room: President Hillary Clinton,” said Sen. John Thune, who endorsed Trump, rescinded his endorsement, and then restored it.

Now Clinton has been stopped, and the task facing Senate Republicans has changed: Trump must be checked — for the good of the country, but also for the good of the Republican Party. If it was Clinton’s success that threatened the GOP in 2016, it is Trump’s corruption, incompetence, and impulsiveness that threatens to destroy it in 2018 and beyond.

A bad candidate can blow a winnable election — which I thought Trump would do. A bad president can destroy a political party — think of what Pete Wilson and Proposition 187 did to California Republicans, or what George W. Bush’s presidency did to Republicans (which includes discrediting the GOP establishment so thoroughly that the Tea Party took over and the base was open to a Trump-like outsider in 2016).

On issue after issue, Trump threatens to do real damage to not just the country but the party, ideology, policies, and ideas to which Senate Republicans have devoted themselves.

A party that fears crony capitalists just elected a crony capitalist

“Crony capitalism corrupts the free market by rewarding political connections over competitive excellence,” said Sen. Mike Lee in April 2014. “It subverts the rule of law by codifying inequality. It undermines social solidarity by pitting citizens against one another, twisting cooperative communities into rival special interests.”

This isn’t just an idiosyncratic crusade of Lee’s — the danger of crony capitalism has become a core tenet for reform-minded conservatives. In this, Lee is joined by both Trump skeptics like Pat Toomey and Ben Sasse and Trump endorsers like Paul Ryan and Marco Rubio.

Meanwhile, Republicans just elected one of the country’s leading crony capitalists to office. Trump bragged during the campaign that he routinely bought off politicians to further his own business interests and has, since winning the election, been brazen in using his newfound power to further his own brand. Nor is Trump shackled by the traditional ideological, temperamental, or institutional restraints that curb politicians’ most transactional tendencies. As Matt Yglesias wrote in an excellent piece on Trump’s corruption:

Commitment to ideological principles normally serves as a form of restraint on a presidential administration — there are some deals a Barack Obama or George W. Bush simply wouldn’t cut. But Trump has no particular ideological fixed points, and has time and again showed his willingness to be creative and make up new policy positions or rationalizations for old ones on the fly. [...] He will be free to operate in a much more opportunistic manner than recent presidents, rewarding friends and supporters and punishing foes without regard to logic or consistency. Kleptocratic government and autocratic government will serve as mutually reinforcing tendencies — businesses that align with Trump will prosper while those that do not will fail, creating a compromised business-political elite that cannot afford to lose power.

Parties are defined by their presidents. If Republicans don’t want their party to become the party of crony capitalism, they have to stop Trump from running an administration powered by payback, favors, and self-interest. And the way to do that is to vote down nominations of loyalists and sycophants, to beat back the legislation that emerges from this side of Trump’s personality, and to investigate and curb corruption within Trump’s network before it blossoms into kleptocracy.

Stopping Trump isn’t just about ideology — it’s about self-interest

The GOP’s Trump skeptics include a number of relative moderates who think immigrants should be treated with respect and compassion, who rely on support from independent and even Democratic voters in their own states, and who will find themselves vulnerable if Trump enrages the Hispanic electorate. John McCain, Lindsey Graham, Susan Collins, Cory Gardner, and Dean Heller all fit this description. It should make them gravely skeptical to see Sen. Jeff Sessions — as ardent an opponent of Hispanic immigrants as exists in the US Senate — ascend to a position as powerful as attorney general.

And then there’s the simple incentive that unites all congressional Republicans: If Trump’s administration collapses beneath scandal, corruption, incompetence, or extremism, they — and their congressional majorities — will pay the price in 2018, while Trump doesn’t face voters again till 2020.

It’s the out-of-power party that typically gains during midterm elections, and waves come when presidents fail. For that reason, if no other, all Republicans should use their power to force Trump to appoint qualified, sensible people to key positions, to keep his conflicts of interest from becoming an endless series of damaging scandals, and to keep him from pushing legislation and priorities that will enrage a nation that mostly didn’t vote for him and mostly dislikes him.

If all this reads as naive — if it seems crazy that Senate Republicans would act as a check against a president of their own party — then remember that this is how it is supposed to work. America’s system of government is built on checks and balances. Congress is meant to restrain the president — one of many safeguards the founders built because they, unlike us, had no trouble believing a demagogue might someday win a presidential election. But that system has broken down, particularly in recent years. Instead of a government defined by competition among branches, we developed a government defined by competition between political parties that occurs across branches.

Trump, however, is a special case: He is not truly of either political party. Presidents are often thought of as “party leaders,” but Trump does not have the relationships, the ideology, the institutional ties, or the deep networks that politicians usually need to win a major party’s nomination. His path to the presidency began with a hostile takeover of the Republican Party’s primary process — a strategy that worked at a moment when the GOP was unpopular, but leaves him with little goodwill to fall back on once he takes office.

None of this is to argue that congressional Republicans should force Trump to govern as a Democrat. Rather, it’s to say congressional Republicans should insist Trump govern as a Republican and as a conservative. Letting the Trump administration spin off into corruption, kleptocracy, and incompetence doesn’t serve the country, or the Republican Party.