Beginning with Ronald Reagan, I have voted Republican in every presidential election since I first became eligible to vote in 1980. I worked in the Reagan and George H. W. Bush administrations and in the White House for George W. Bush as a speechwriter and adviser. I have also worked for Republican presidential campaigns, although not this time around.

Despite this history, and in important ways because of it, I will not vote for Donald Trump if he wins the Republican nomination.

I should add that neither could I vote in good conscience for Hillary Clinton or any of the other Democrats running for president, since they oppose many of the things I have stood for in my career as a conservative — and, in the case of Mrs. Clinton, because I consider her an ethical wreck. If Mr. Trump and Mrs. Clinton were the Republican and Democratic nominees, I would prefer to vote for a responsible third-party alternative; absent that option, I would simply not cast a ballot for president. A lot of Republicans, I suspect, would do the same.

There are many reasons to abstain from voting for Mr. Trump if he is nominated, starting with the fact that he would be the most unqualified president in American history. Every one of our 44 presidents has had either government or military experience before being sworn in. Mr. Trump, a real estate mogul and former reality-television star, hasn’t served a day in public office or the armed forces.