Poverty may be a central issue in Mitt Romney's campaign if he chooses to run for president a third time. The former Massachusetts governor's allies are suggesting he's learned his lesson after footage of him making disparaging comments about the "47 percent" became public in 2012.

That 47 percent, in case you've forgotten, was the approximate fraction of households that paid no federal income tax in 2009, according to the Tax Policy Center's Roberton Williams. Romney said that these people would not vote for him. "I'll never convince them they should take personal responsibility and care for their lives," he said.

Subsequent estimates put the figure for 2009 at 50 percent, but in 2012, when Romney spoke, it had already declined to about 42 percent. The center projects that the number will fall to 40 percent this year and further in the future, as shown in the chart below.

Yet these numbers have never revealed much about who is and isn't paying taxes, said Williams, an economist who never expected to create a controversy when he published a short paper with his estimate that 47 percent of households do not pay federal income tax.

"It's an interesting number. It's not that useful," said Williams, who recalled being introduced as "Mr. 47 Percent" on a cable news show.

First, most of those who don't pay income taxes still work and draw a paycheck, so they do pay federal payroll taxes. Between payroll and income taxes, only 23 percent of households paid nothing to the federal government or received a refund.

There are several reasons why this group might not be paying federal taxes. They could be students, or they could be retirees, whose Social Security payments are usually not taxed. They could be disabled, or they could be out of work.

Or, they could be working, but making almost nothing. The personal exemption and the standard deduction -- two basic provisions of the tax code -- protect the first $26,400 earned by a family of four from taxation so that people without means aren't giving what they have to Uncle Sam.

Those who are making more than that might still not pay income tax because of the earned income tax credit, which offers a monetary bonus to workers at tax time as a way of encouraging people to get jobs, and the child tax credit, which helps kids who might otherwise be living in poverty. Many conservatives have championed these provisions in the tax code.

The number of people paying no income tax reached such high levels after the financial crisis because many people lost their jobs and because of temporary tax cuts Congress and President Obama enacted to stimulate the economy. Williams predicts the number will continue to decline over time as the economy improves.

Not only is this aggregate number falling, but individuals' financial positions are also changing every day. As a result, paying no income tax is probably a temporary condition for most people. They don't pay when they're young and in school, after they've retired, or when they've just been laid off. Some 61 percent of those who claim the earned income tax credit only do so for less than two years.

Meanwhile, while many people who are less well-off do not pay federal income and payroll tax, they are still giving up a disproportionate share of their income to federal excise taxes (such as the gas tax) and to state and local taxes.