Fordham University political scientist Costas Panagopoulos, analyzing the same data, found that Rasmussen, SurveyUSA and Public Policy Polling produced results close to the final outcome. Mason-Dixon Polling & Research and YouGov were among the further off-course.

But during the 2010 elections, Rasmussen missed the mark by a far wider median margin than did SurveyUSA, YouGov and PPP, according to Journal analysis of data compiled by NCPP from Senate polls. The Quinnipiac University Polling Institute did better than all of them.

Gallup has received a lot of scrutiny this year as its tracking poll over the past month gave a large lead to Mr. Romney. Its final poll, released Monday afternoon, shows Mr. Romney leading, 49% to 48%.

Lee Miringoff, director of the Marist Poll, which is conducting polls of battleground states this year for the Journal and NBC News, notes that pollsters who complete their last poll closer to Election Day have an advantage in predicting the final result. Assessments of accuracy also can differ depending on whether they measure how close a pollster got to the candidates’ final vote totals or their margins of victory.