Using independent polling data, the president's advisors said that Romney had enjoyed barely any post-announcement "bounce" in the polls since naming Ryan on Saturday, and compared him to the failed pick of Sarah Palin in 2008.

Obama's pollster Joel Benenson told reporters that since 1996, new vice presidential nominees generally gave their new bosses a bump of about five percent in Gallup polls.

"If you contrast that with where Congressman Ryan is, he's had virtually no impact on Romney's position in the polls," Benenson said.

Benenson cited Gallup's daily tracking poll in which Romney had picked up only one point since naming Ryan, and said he had lost a point in the Rasmussen Reports tracking survey.

Before the blizzard of media coverage unleashed by the pick of Ryan, a conservative Republican congressman, Romney led Obama in the Gallup tracking poll by 46 to 45 percent. Four days later, he was up by only 47 to 45 percent.

According to the Gallup poll, former Republican nominee John McCain got a two-point bounce with his pick of Palin, the then unknown governor of Alaska, in 2008, a selection which turned into a liability for his campaign.

The biggest poll bounces for vice presidential picks were nine percent, for Jack Kemp, picked by Bob Dole as his Republican running mate in 1996, and Joe Lieberman, who gave a five percent lift to Al Gore's Democratic ticket in 2000.

Both Dole and Gore went on to lose the election in November, however, supporting the conventional wisdom that vice presidential nominees have a negligible impact on how a presidential candidate is judged.