Overall, 80 percent of pre-convention leaders claimed the White House, Gallup said Monday.
Pre-convention polls help in terms of simply forecasting which candidate would win the general election, the Princeton, N.J., polling agency said. However, when the pre-convention polls are close -- as they have been for President Obama and presumptive Republican presidential nominee Mitt Romney this year -- historical data indicated the leading candidate is more likely to be the eventual winner.
In Gallup's seven-day rolling average, Romney and Obama were tied at 46 percent.
In tight pre-election polls, Gallup said the margins in the final popular-vote tally in which the leading candidate lost could result from several factors -- the so-called convention bounce, the debates, the effect of the campaigns, undecided voters finally choosing a side, and which registered voters actually vote in the election.
Obama and Romney have been closely matched over the last four months, Gallup said, resembling pre-convention polls in 1960, 1968, 1980, 2004 and 2008 -- all elections in which pre-convention polling showed one candidate leading the other by no more than 4 percentage points. In four of those five elections, with 2004 the exception, the candidate with a slim lead before the first convention won on Election Day.
The Republican National Convention is this week in Tampa, Fla. The Democratic National Convention begins next week in Charlotte, N.C.
Notable deaths of 2014 [PHOTOS]