Robert Kagan is a senior fellow at the Brookings Institution and a contributing columnist for The Post.

Will the Republican Party that made Donald Trump its prospective nominee protect us from Trump when he is president? Even as they call him a “textbook” racist and acknowledge his scant regard for the rule of law, Republican leaders assure voters that the U.S. system of checks and balances will contain their candidate’s authoritarian impulses. Congress and the judicial system will keep Trump under control.

History and recent events suggest that is a risky proposition. Inflamed popular passions and overreaching presidents have at times not been checked. Presidents have ignored Supreme Court rulings; and the Alien and Sedition Acts of 1798 and 1918, Jim Crow, the mistreatment of German Americans during World War I and of U.S. citizens and noncitizens of Japanese descent during World War II, and the investigations of Sen. Joseph McCarthy all showed how a frightened, angry or simply bigoted majority could deprive individuals of their rights despite the Constitution’s checks and balances. That those rights were eventually restored is no cause for satisfaction: The damage done was permanent.

Nor is it reason for complacency, especially now. Never before has a presidential candidate given more reason to fear that he will run roughshod over democratic institutions and abuse the vast powers of the presidency for personal ends. Not a week goes by without Trump providing fresh evidence that he neither understands nor values our political and legal systems but rather sees them as tools to be manipulated or obstacles to be overcome. He threatens to change libel laws to go after media outlets. He attacks federal judges as unfit on grounds of ethnic background. He promises, if elected, to have his attorney general launch investigations of his political opponents. In the past, Americans did not know as they voted that their presidents would seek to abuse their executive powers. This time, and indeed for the first time ever, they do.

Laughing, House Speaker Paul Ryan (R-Wis.) responded to a reporter's question about GOP presidential candidate Donald Trump and the separation of powers at a news conference June 16. He also spoke about Trump's decision to revoke The Washington Post's media credentials. (Reuters)

As for the Republicans, after being unwilling to stop Trump from clinching the nomination, and being unwilling to abandon him after deciding that he is a racist, they want us to trust that they will be willing to fight him once he becomes president. More likely the opposite will be true.

Consider the reasons Republicans support Trump today. The first is party interest. Trump was chosen by the voters in a legitimate race and according to the rules of the Republican primary process. To abandon him, they fear, would destroy the party. Moreover, it would hand a victory to the “Obama-Clinton-Sanders” Democrats, who some Republicans insist would be an even bigger disaster. Finally, Republicans up for election fear that if they oppose Trump and anger his supporters, they will face dangerous primary challenges or lose in the general election.

Which of these motives will disappear once Trump becomes president? He will still be the Republican Party’s legitimately chosen leader, as well as the legitimately elected president. The election cycle doesn’t end in November. To oppose Trump as president will be even more contrary to the party’s interests than it is now. Will Republicans line up with Democrats to vote against Trump-inspired legislation — to ban Muslims from entering the country, for instance, or to deport 11 million illegal immigrants? To do so would only hand the opposition major political victories, setting the stage for Democratic congressional gains in 2018. Party interests will require that the party support its president.

Even in the unlikely event that some brave Republicans did act in ways contrary to the interests of their party, what would their constituents say? Two years ago, Republican voters threw out the House majority leader because he was, in their view, too willing to compromise. Would they feel differently if Republicans voted with Democrats against the Republican in the White House? And imagine how a President Trump would respond to a rebellion in the ranks. Trump already has a record of vindictiveness against those who resist him, including within his own party — would he forget the person who said he engaged in “textbook” racism? — and Republicans already have a record of caving.

In short, anyone looking to Congress to curb the excesses of a President Trump will have to count on the Democrats. Is that the Republican message: Don’t worry about Trump, Democrats will protect you?

To hope that the judicial system will check Trump may be equally fanciful. The courts have historically been reluctant to challenge the president on actions they deem related to national security. Trump himself has noted that he will have the authority to close the borders to certain groups. And as Brookings Institution legal scholar Benjamin Wittes points out, the Justice Department is always vulnerable if a president wants to manipulate it for his or her own purposes. Trump has already said that if elected he will have his attorney general look into the matter of Hillary Clinton’s emails. Republicans and conservatives may delight to hear it, but what Trump can do to Democratic opponents he can also do to Republicans who defy him.

Wittes makes the point that what keeps the attorney general and the Justice Department from abusing power is not the law so much as a respect for “norms and human and institutional decency.” In fact, this is true of our entire constitutional system. The checks and balances do not automatically snap into action whenever a president overreaches. The people and their representatives have to make the system work. It is a never-ending battle. As the political scientist Edward Corwin once put it, the Constitution is an “invitation to struggle,” but our system relies on all three branches waging that struggle in a democratic spirit. No one knew better than the founders that the system they designed was neither foolproof nor tyrant-proof. The people had to make good decisions, including choosing political leaders who respected the system and the rights it safeguards.

Here are five GOP lawmakers who have taken issue with the ways their party's presidential candidate has reacted to the massacre in Orlando. (Sarah Parnass/The Washington Post)

As Benjamin Franklin said, “a Republic, if you can keep it.” Today, Americans can’t simply rely on the system to save them from the possibility of a fascist president. And they certainly can’t count on the Republicans who produced this threat in the first place. They will have to shoulder that responsibility themselves, in the voting booth.