Unless you are Ronald Reagan running for re-election (“It’s morning again in America”), most competitive presidential campaigns are about a single objective: making the race a referendum on the opponent, particularly if he or she is a long-time politician who has high negatives.

That’s how Barack Obama won a second term. He defined Mitt Romney and ran against that caricature he created (with Romney’s help, of course).

But whether it’s because he really doesn’t understand campaigns, or more likely, that his obvious narcissism makes it impossible for him to see that any topic could be more interesting than himself, Donald Trump continues to make the 2016 election a referendum on his accomplishments, his past statements and his beliefs.

That braggadocio may have worked in a crowded primary with weaker-than-anticipated opponents and a GOP grassroots that wanted red meat and a pure outsider with a flair for entertainment. But it is much less likely to work in a two-way sprint for the White House.

It’s not as if Hillary Clinton is a politician without political warts. With negative personal ratings in the mid-50s, she looks like an easy target for serious, well-planned, focused attacks on her judgment, character and policies.

Polls show voters harbor real doubts about how forthright she has been about her emails, how successful her years at the State Department were, and whether a professional politician is best suited to run the country for the next four years.

Clinton is so vulnerable that against an uncontroversial Republican opponent who could pass the presidential smell test, Clinton probably would be a narrow underdog in November. After all, Ohio Gov. John Kasich held a lead against her in national ballot tests right up to the time that he left the GOP race with a single primary win under his belt.

Kasich’s strength against Clinton wasn’t that he was so well-known, well-respected and well-liked. It was that he was not Hillary Clinton. Voters knew her political baggage but not his. In other words, the former secretary of state couldn’t beat a placebo.

But instead of trying to be less controversial, more welcoming to friends and foes alike, and less of an issue in the election, Trump has done the exact opposite.

Time and again, he has kept the focus on himself by doubling down on defending Trump University and raising questions about the integrity of a federal judge handling the case, by attacking the Hispanic Republican governor of New Mexico and even by criticizing a supporter, former Speaker Newt Gingrich.

Trump’s language continues to be coarse and undignified, his sentences rarely complete and often illogical. His temperament is “strong?” He is angry at the media because reporters don’t compliment him? Hillary Clinton is “guilty” even before she has been indicted or convicted?

All of this has made Trump an easy target for both Clinton and the national media.

Republican nominees inevitably prefer to talk about the size of government, taxes, spending, deficits, over-regulation and defense/national security. They talk about cultural issues (e.g., abortion, same-sex marriage) to their base, unless they find a larger social issue, such as welfare reform, that has broader appeal.

Democrats, on the other hand, generally prefer to talk about wages, infrastructure, economic and social inequality, civil rights, the environment, abuses by business and international cooperation.

Both parties’ nominees invariably talk about jobs and the economy, though generally one party is on the attack, blaming the other for failing to “get the economy going” and failing to produce more and better-paying jobs.

Some election political and economic environments favor Republicans – 1980 is an obvious example – while others, including 2008, favor Democrats.

This year, the Democrats will nominate a former member of the current administration, and any savvy GOP presidential nominee would try to make the 2016 election a referendum on Clinton and even on Obama, though his job approval now stands around the 50 percent mark.

But instead of offering detailed criticisms of Clinton’s record in office or of the Obama Administration’s performance, Trump invariably prefers to make himself the issue. Instead of using the new May jobs numbers to rail against the Obama administration’s failings, Trump spent a weekend talking about the judge in the Trump University case and inviting his critics – and even some friends – to skewer him repeatedly.

Call him flamboyant or unhinged, unconventional or unschooled in campaigns, Trump seems unlikely to change his demeanor and unwilling to do the work necessary to take advantage of Clinton’s many vulnerabilities. And yet, even now he is in range of Clinton if the polls are correct.

On MSNBC’s “Morning Joe” on Tuesday, Tennessee GOP Sen. Bob Corker insisted that there is still time for Trump to change his message and style, to “pivot to the general election.”

Really? If a new Donald Trump appeared tomorrow (maybe a bit like the one who spoke on Tuesday night), would anyone seriously believe that that would be the “real” Trump?

It’s a lot easier to call your opponent names and brag about your own successes than it is to prosecute a political campaign that exposes your opponent’s weaknesses and holds her responsible for the last eight years.

But for Trump, nothing seems to be as much fun as talking about himself.