The public in 1948 remembered fondly the farm supports, banking regulations, and social welfare provisions of the New Deal, to say nothing of success in World War II, and Truman could draw upon this deep reservoir of trust in what the government had done for the country over the last two decades. Such trust does not exist today; instead, the number of people who are skeptical of the government’s competence is at an all-time high. Worse, the main achievement of the current administration—Obamacare—is deeply unpopular, favored by less than 40 percent in the latest RealClearPolitics average of recent polling. And for good reason: Credible reports from nonpartisan agencies like the Congressional Budget Office and the Centers for Medicare and Medicaid Services predict that under Obamacare millions of people will lose their current insurance, pay higher premiums, and may even see their doctors stop accepting Medicare patients.

In other words, Truman could argue in 1948 that a vote for the Republicans would threaten the social welfare system the country knew and approved; but in 2012 the Republican nominee will be the one who can argue that a vote for Obama will endanger that system.

So, if President Obama cannot look to Truman for a model, where does that leave him? Essentially, he’s in uncharted territory. No incumbent president in the modern era has faced the kinds of trouble he now does—a weak economy combined with an exceedingly unpopular legislative record—and still won reelection.