In January, one man will stand on stage at the Capitol and take an oath to “preserve, protect and defend the Constitution of the United States.” For most of American history, this occasion has been a matter of ceremony, with little doubt that the president would fulfill his oath, regardless of which political party he represented. But this time we can’t afford to take it for granted.

As a Marine, I took a similar oath: to “support and defend the Constitution of the United States against all enemies, foreign and domestic.” I took it seriously, and I live by it today. That’s why I was so appalled last year when I heard former President Donald Trump’s lawyers denied that he had ever pledged to support the Constitution, arguing presidents are not held to that commitment.

This was particularly galling because, in 2016, I was one of the millions of Americans who voted for Trump. I was never a Trump fanatic — I didn’t have the hat, or a Trump flag in my yard. But between Trump and Hillary Clinton, I viewed him as the better of two very flawed candidates and thought his outsider profile could bring much-needed change to Washington.

My hope didn’t last long. On day one, when Trump instructed his press secretary to lie to the world about the size of his inauguration crowd, I knew that I — and the country at large — had made a terrible mistake. It wasn’t long before he was threatening nuclear war against foreign dictators, lining his pockets with taxpayer dollars, mismanaging a pandemic, and telling a grand total of 30,573 lies throughout his presidency, according to The Washington Post.

As bad as the lies and the incompetence were, the bullying was worse. In the military, they teach you to build up your comrades. Tough love is fine — even encouraged — but it should strengthen the individual and the unit. Trump’s approach is antithetical to this. His instinct is to demean and dehumanize others in the coarsest possible terms. Besides setting a dismal example, this degrades our politics, encouraging everyone to be their worst selves.

There’s no perfect politician. President Joe Biden is living proof of that. I happily supported Biden in 2020, eager to do whatever it took to get Trump out of office. I have no regrets, and I will support him again in 2024. Biden has made his share of policy missteps, including abandoning America’s Afghan allies and over-stimulating the economy, which led to inflation. The difference between Biden and Trump is that these policy flubs can be rectified, while Trump’s all-out assault on the truth and democracy leaves permanent scars.

Nowhere is this more evident than in Trump’s behavior after the 2020 election, culminating in the violence of January 6. He refused to accept that he lost a free and fair election, encouraging his followers to believe the Big Lie that the election was stolen. I watched as lies and conspiracy theories about the election consumed people in my own life. The irresponsibility on display from national leaders who knew better, and chose to do nothing about it, is still stunning.

When the courts foreclosed Trump’s legal path to overturning the election, he whipped his followers into a frenzy and incited their attack on the Capitol, in one of the darkest and most shameful days in our nation’s history. That thousands of troops had to be deployed to safeguard Washington in the weeks after the insurrection is a permanent stain on the record of our democracy. When we put the uniform on, few of us expected it would be to protect the nation from its own citizens.

Many veterans have long supported the GOP because of its reverence for military service and support for the armed forces. But Trump is another story. He has called fallen soldiers “suckers” and “losers,” refused to honor them and insulted the service of national heroes like the late Sen. John McCain. He’s the only politician I’ve known to pick a fight with a Gold Star family. As long as Trump is the figurehead of the Republican Party, its pro-military rhetoric will ring hollow.

Walking away from one’s “team” during election season can be difficult. For many, it can seem like a betrayal to support Biden and the Democrats. That’s why I’m part of a national campaign called Republican Voters Against Trump, which amplifies the voices of former Trump voters who refuse to back him in November — to show just how many of us there are out there. Not all of us are sold on Biden, but we are united in refusing to support Trump on principle.

I had seen enough on day one of the Trump presidency. Years later, I feel more strongly what I knew then to be true: He is unfit for the presidency. His egomania, bullying, disregard for basic decency and open contempt for the military and our veterans render him unqualified. It’s true, Biden is a flawed candidate. But I have no qualms about supporting him in November, because it’s what my oath demands of me.

Mike Kabatt lives in Maryland and is a participant in Republican Voters Against Trump, a nationwide campaign of former Trump voters who are not supporting him in 2024.