“Without a doubt President Obama inherited a difficult situation. Here’s the problem. He made it worse,” Paul Ryan, the Republican vice presidential candidate, has been saying in his stump speech.

Ryan’s statement consists of two parts; the first is gross understatement, the second gross misstatement. It is the misstatement that is the essence of the case Republicans are putting before American voters: That President Obama has made the economy worse. Getting voters to believe that assertion is probably the Republicans’ only hope of winning the election.

In the latest poll (a Wall Street Journal and NBC News survey released on Tuesday), respondents favor President Obama over Gov. Romney — generally by wide margins — in almost every category other than improving the economy. On “caring about average people,” for example, Obama is favored by an extraordinary margin to 52 percent to 30 percent.

So the argument that President Obama has made the economy worse is not only central to the Republicans’ case, it’s pretty much all they have. But the facts do not support their claims.

The following graph, which I put together using data from the Bureau of Labor Statistics, traces the annual rate of job creation under Democratic and Republican administrations from Harry Truman to Obama — with 2009 separated out as a year in which any fair-minded person would agree that the “difficult situation” Obama inherited was the main driving force. It dramatically illustrates just how wrong Ryan’s assertion is.

Source: Wall Street Journal, Bureau of Labor Statistics

Republicans who are wondering how Obama could be polling as well as he is with unemployment above 8 percent might want to take a look at this graph. It makes two points very clear.

First, jobs have historically, without exception, been created at a much faster rate under Democratic presidents than under Republicans.

Look at the 64-year period from the start of Harry Truman’s presidency to the end of that of George W. Bush (1945-2009).

During the 28 years of Democratic administrations in that period, 57.5 million new jobs were created, an average of 2.05 million per year.

During the 36 years of Republican administrations in that period, 36.2 million new jobs were created, an average of 1.0 million per year.

The bottom line is that over the 64 years leading up to the inauguration of President Obama, jobs were created more than twice as fast under Democrats as they were under Republicans.

Even a glance at the graph shows that Barack Obama and his policies did not make the “difficult” — disastrous would be a more accurate term — situation he inherited worse.

Here is a second graphic illustration of what has happened in job creation under George W. Bush and Barack Obama. It also uses data from the Bureau of Labor Statistics.

Source: Bureau of Labor Statistics

In the eighteen months from the beginning of 2008 through the middle of 2009, a period fully shaped by the Bush economic program to which Republicans now want to return, (but before the Obama stimulus had a chance to take effect), approximately 7.5 million jobs were lost.

Over the most recent 18 months of the Obama administration, approximately 2.8 million jobs have been added.

That means that the average monthly job loss during the “difficult situation” before Obama’s policies took effect was 417,000. Over the last year-and-a-half, the average monthly job gain has been 155,000.

If Rep. Ryan and Gov. Romney see that as making a bad situation worse, it should tell us something about their “vision.”

Robert S. McElvaine is a professor of history at Millsaps College. His most recent book is the 25th anniversary edition of “The Great Depression.”