To see the magnitude of the challenge facing President Obama when he speaks tonight, one need only look at his address to the 2008 Democratic National Convention. In it, he laid out clear metrics for how his presidency should be judged – and by his own standards he has failed.

In 2008, Obama delivered a devastating critique of the state of the economy, with words that could just as easily describe the state of the economy today: “Tonight, more Americans are out of work and more are working harder for less. More of you have lost your homes and even more are watching your home values plummet. More of you have cars you can’t afford to drive, credit card bills you can’t afford to pay, and tuition that’s beyond your reach.” Of those responsible for this disastrous state of affairs, Obama declared, “It’s time for them to own their failure.”

He then set clear benchmarks by which we could determine whether his administration has been a success. “We Democrats have a very different measure of what constitutes progress in this country,” Obama told the assembled delegates. “We measure progress by how many people can find a job that pays the mortgage; whether you can put a little extra money away at the end of each month so you can someday watch your child receive her college diploma. We measure progress in the 23 million new jobs that were created when Bill Clinton was president — when the average American family saw its income go up $7,500 instead of down $2,000, like it has under George W. Bush.”

So how does Obama’s record stack up against the standard he set himself in 2008? Obama said a decline in family income of $2,000 over eight years was a dismal failure. Well under his leadership, the average American family saw its income go down by almost $4,000 in just three years — from $54,797 when Obama took office in the first quarter of 2009 to just $50,964 in the first quarter of 2012. Instead of 23 million new jobs, Obama is the only president since World War II to have presided over a net job loss. The U.S. is now in its 42nd consecutive month of above-8 percent unemployment — the longest stretch of sustained high unemployment since the Great Depression.

In his 2008 address, Obama said Republicans were launching negative attacks on his character to distract attention from their dismal economic record. “If you don’t have any fresh ideas, then you use stale tactics to scare the voters. If you don’t have a record to run on, then you paint your opponent as someone people should run from. You make a big election about small things,” Obama said. In 2012, Obama’s campaign strategy is centered almost exclusively on painting his opponent as someone people should run from — a heartless corporate raider, and possible felon, who killed a woman. The candidate making a big election small is the one who indicted others for doing so four years ago.

Four years ago, Obama said our economic crisis was “a direct result of a broken politics in Washington and the failed policies of George W. Bush.” Well, Obama’s been in charge for almost four years now, and our politics are as broken as ever. And regardless of the tough hand he was dealt, by a margin of 56 percent to 35 percent Americans say his policies are making things worse, not better.

This means that if Obama’s message tonight is “stay the course” — that his policies are working and he just needs more time — his speech will backfire. “Stay the course” didn’t work for George W. Bush in 2006, when Americans knew our strategy in Iraq was failing. It won’t work any better for this president in 2012, when large majorities believe Obama’s economic strategy is failing. If Obama wants voters to give him a second chance, he needs to lay out a vision for “change” — and show the American people how a second Obama term would be different from the first. If he doesn’t, they are unlikely to give him one. If he promises more of the same, Americans may well heed his advice from four years ago — and make him own his own failure.

Marc A. Thiessen, a fellow with the American Enterprise Institute, writes a weekly online column for The Post.