On Thursday, Joe Biden will deliver a primetime speech outside the old Independence Hall, where the framers of the constitution met 235 years ago to establish the basic rules of our democratic form of government.

His speech will focus on what the White House describes as the “battle for the soul of the nation” – the fight to protect that democracy.

The battle is already under way. A week after a team of FBI agents descended on his residence in Florida, Trump warned “people are so angry at what is taking place” that if the “temperature” isn’t brought down “terrible things are going to happen”.

Yet Trump and his Republican allies are doing all they can to increase the temperature. Last Sunday, Senator Lindsey Graham warned of “riots in the streets” if Trump is prosecuted.

Trump spent much of Tuesday morning reposting messages from known proponents of the QAnon conspiracy theory and from 4chan, an anonymous message platform where threats of violence often bloom.

Several of Trump’s reposts were direct provocations, such as a photograph of President Biden, Kamala Harris and Nancy Pelosi with their faces obscured by the words: “Your enemy is not in Russia.”

Online threats are escalating against public servants. Bruce Reinhart, the federal magistrate judge who approved the warrant to search Mar-a-Lago, has been targeted with messages threatening him and his family.

How to respond to this lawlessness? With bold and unwavering law enforcement.

If Trump has broken the law – by attempting a coup, by instigating an assault on the US Capitol, by making off with troves of top-secret documents – he must be prosecuted. If found guilty, he must be penalized, including by prison.

Yes, such prosecutions might increase tensions and divisions in the short term. They might provoke additional violence.

But a failure to uphold the laws of the United States would be far more damaging in the longer term. It would undermine our system of government and the credibility of that system – more directly and irreparably than Trump has already done.

Not holding a former president accountable for gross acts of criminality will invite ever more criminality from future presidents and lawmakers.

It is also important for all those in public life who believe in democracy to call out what the Republican party is doing and what it has become: not just its embrace of Trump’s big lie but its moves toward voter suppression, takeovers of the machinery of elections, ending of reproductive rights, book bans, restrictions on what can be taught in classrooms, racism and assaults on LGBTQ people.

While today’s Republican party does not have its own paramilitary, such as the Nazi’s Brownshirts, Republicans are effectively outsourcing these activities to violent fringe groups such as the Proud Boys, Oath Keepers and others who descended on the Capitol on January 6 2021 and who continue to threaten violence.

With the notable and noble exceptions of Liz Cheney and a few other courageous Republicans – most of whom are being purged – the Republican party is rapidly morphing into an anti-democracy movement.

The essential political choice in America, therefore, is no longer Democrat or Republican, left or right, liberal or conservative.

It is democracy or authoritarian fascism.

There can be no compromise between these two – no halfway point, no “moderate middle”, no “balance”. To come down squarely on the side of democracy is not to be “partisan”. It is to be patriotic.

Yet Democrats cannot and must not take on this battle alone. They must seek common ground with independents and whatever reasonable Republicans remain.

We must continue to appeal to truth, facts, logic, and common sense. We must be unwavering in our commitment to the constitution and the rule of law.

We must be clear and courageous in exposing the authoritarian fascist direction the Republican party has now chosen, and the dangers this poses to America and the world.

It is also important for Democrats to recognize – and to take bold action against – the threat to democracy posed by big money from large corporations and the super-wealthy: record amounts of campaign funding inundating and distorting our politics, serving the moneyed interests rather than the common good.

The two threats – one, from an increasingly authoritarian-fascist Republican party; the second, from ever-larger amounts of corporate and billionaire money in our campaigns and elections – are two sides of the same coin.

Americans who know the system is rigged against them and in favor of moneyed interests are more likely to give up on democracy and embrace an authoritarian fascist demagogue who pretends to be on “their side”.

The battle to preserve and protect democracy is the most important battle of our lifetimes. If we win, there is nothing we cannot achieve. If we lose, there is nothing we can achieve.

• Robert Reich, a former US secretary of labor, is professor of public policy at the University of California, Berkeley and the author of Saving Capitalism: For the Many, Not the Few and The Common Good. His new book, The System: Who Rigged It, How We Fix It, is out now. He is a Guardian US columnist. His newsletter is at robertreich.substack.com