In the old days, Labor Day marked the start of the presidential race. This year Democrats won’t officially choose their candidate — our guess is Barack Obama — until two days after Labor Day.

The campaign between the incumbent president and his putative Republican challenger Mitt Romney began to crystallize last week. The 2012 campaign, more than any since 1932, will turn on the economy. Last week, the stark differences between the two candidates on the issue began to form around one fundamental policy difference.

Obama kicked it off on Monday by inviting a group of middle-class families to the East Room of the White House and telling them, “Let’s not hold the vast majority of Americans and our entire economy hostage while we debate the merits of another tax cut for the wealthy.”

The president wants to allow the top marginal tax rates to return to where they were before President George W. Bush and Congress reduced them — “temporarily” — in 2001. The effect would be that American households would begin paying higher rates on every dollar of taxable income above $250,000. Only about 2 percent of households earn that much.

This reprises the arguments of 2010, when Obama agreed to extend all of the Bush tax cuts, including those for the wealthiest 2 percent. But unless Congress acts before year’s end, everyone’s tax rates will return to where they were during the Clinton administration. Obama’s argument is that to protect the richest 2 percent, Republicans are willing to allow everyone’s taxes to go up.

This is where the president has chosen to make his stand. He spent much of the rest of the week campaigning on the issue.

Romney responded immediately, saying on a radio show that his own tax plan “brings down the rate of taxation for individuals and for small business and for businesses of all kinds” while limiting tax breaks used by wealthier taxpayers.

This has been his plan since the primaries. He has not yet been specific about the breaks he would eliminate.

Regardless of foreign affairs, wars and other issues, the tax issue is the whole ballgame.

It speaks to Romney’s insistence — despite a new Congressional Budget Office report that federal taxes are at an all-time low — that taxes are too high and sometimes confiscatory.

It speaks to the small-business owners who drive most of the job creation in this country while the wealthy park their money in hedge funds and the Cayman Islands. Only 2.5 percent of actual small-business owners would face higher taxes under the president’s plan.

It speaks to the Republicans’ theological belief — despite negative job growth after the Bush tax cuts — that low taxes yield more jobs.

It speaks to issues like the fact that the richest 400 American families are now worth more than the poorest 150 million.

It speaks to budget problems and fixing the deficit. The two-year extension of tax cuts for the wealthiest 2 percent cost the treasury $80 billion. Over 10 years, Obama’s tax proposals — which include raising the tax on capital gains from 15 percent to 20 percent — would raise $850 billion.

Facts and fairness are on the president’s side. Romney counters with money and mythology. This is the choice. The game is on.