Perhaps it’s a risk for any incumbent Democratic president running for re-election, but President Obama now finds himself being compared by Republicans to former President Jimmy Carter.

“Obama’s America - Malazy,” trumpets a new ad by the National Republican Senatorial Committee, combining Mr. Carter’s “malaise” speech with Mr. Obama’s recent characterization of America as “lazy” in the pursuit of foreign investors. The ad is titled “Two presidents, one excuse.”

It comes on the heels of polling that showed Mr. Obama’s job-approval rating of 43 percent is lower than Mr. Carter’s rating of 51 percent at the same point during his presidency in 1979. Mr. Obama’s popularity has never reached the depths of Mr. Carter’s low point of 28 percent, however.

“President Obama’s poll numbers are worse than Jimmy Carter’s poll numbers. For the first time in his presidency, he’s waking up this morning wishing that his poll numbers were as good as Jimmy Carter‘s,” Reince Priebus, chairman of the Republican National Committee, said Sunday on NBC’s “Meet the Press.”

Former GOP presidential candidate Tim Pawlenty, now co-chairman of Mitt Romney’s campaign, last week called Mr. Obama’s performance “stumbling, bumbling, ineffective” and reminiscent of the nation’s 39th president.

Mr. Carter is generally recognized as one of the worst presidents of the past half-century, hence the GOP’s eagerness to promote the comparison...