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.