The big topic over the next few weeks will be the post-convention bumps for each candidate in the presidential race.  Will Mitt Romney win some momentum going into September, or will Barack Obama reverse a months-long slide?  Two new polls out today suggest that all sides may need to manage expectations.  First, Gallup’s polling history shows that four times out of five, the polling leader before the convention ends up winning the race in the end:

 As the 2012 presidential conventions get underway in Tampa, Fla., a Gallup analysis of 15 elections from 1952 to 2008 shows that in all but three instances — 1988, 1992, and 2004 — the candidate leading in the Gallup poll conducted just prior to the first convention (the “pre-convention poll”) has won the November election.

Pre-convention polls are not good predictors of a candidate’s final vote share, but they are useful in terms of simply forecasting which candidate will win the election. Overall, 80% of the pre-convention leaders went on to become president, although that success percentage figure includes the disputed 2000 election, in which George W. Bush was elected without winning the popular vote.

Barack Obama and Mitt Romney have been tied or very close in recent  Gallup Daily tracking averages. Gallup’s final pre-convention standing of the two candidates will be based on the Aug. 20-26 average and posted Monday afternoon on Gallup.com. If either candidate is ahead in that average, the historical data outlined below suggest that candidate is more likely to be the eventual winner, although close races such as this one suggest more potential for exceptions to that pattern.

Interestingly, this has been true whether the spread has been wide, such as 1996’s 26-point margin for Bill Clinton, or narrow, as it was with Ronald Reagan’s 3-point edge in 1980 over Jimmy Carter — both in the 30s at that time.  The 1992 exception probably has two explanations: first, it was a three-way race that Clinton won with just 42% of the popular vote, and the Pat Buchanan speech at the 1992 GOP convention was widely believed to have damaged George H. W. Bush.

In a race this close, though, one might expect to see more impact from the conventions … if they get significant voter attention.  Rasmussen’s poll today shows that few expect to watch most of the convention, and fewer independents than anyone else:

Most voters won’t be watching much of the upcoming national political conventions, and over one-third of independent voters plan to tune them out completely.

The latest Rasmussen Reports national telephone survey finds that just 11% of Likely Voters plan to watch all of the GOP convention and another 16% who will watch most. A plurality (44%) expects to watch some of it, and 24% more won’t watch any of the GOP convention held in Tampa, Florida. (To see survey question wording, click here.)

Similarly, only 13% who intend to watch all of the September 4-6 Democratic National Convention in Charlotte, North Carolina. Another 14% who will watch most of it. Thirty-nine percent (39%) will watch just some of the Democratic convention, and 30% plan to ignore it.

Predictably, 90% of Republican and 81% of Democrats intend to watch at least some of their respective party conventions. But just 16% of voters not affiliated with either of the major parties plan to watch most of the GOP convention, and 21% of these voters say the same about the Democratic convention.

Not surprisingly, the younger voters tend to be, the less interested they are in the conventions.  Only 12% of seniors plan to watch none of the Republican convention and 22% none of the Democratic bash, either.  However, those numbers increase slightly for those between 40-64 (14% and 26%, respectively) and jump way up for those between 18-39 (43% and 37%, respectively).  That’s the highest demo for diffidence rate among gender, age, race, or party demos. Furthermore, almost as many voters believe there is too much convention coverage already (35%) than think it about right (40%), and we haven’t even opened either convention yet.

Speaking of gender demos, though, there is one interesting finding.  While there is an eight-point gap among men not planning to watch any of the Republican and Democratic conventions in favor of the GOP (25/33 respectively), there is a +4 edge for the GOP among women, too — 23/27.  That’s within the MOE, but it shows that the “war on women” rhetoric has not driven women off from interest in the Republican Party and its nominee.

Addendum: USA Today will be featuring a number of Republican voices this week to discuss and analyze the convention.  They’re also looking for bloggers to contribute to their forum.