Community organizers and residents of South Los Angeles and throughout the country are excited that Barack Obama has built a significant lead in this historic presidential election. But victory for Obama is still an uphill struggle, as he and his supporters must overcome the Republican biases of our electoral system if they are to reach the mountain top.

Even a normally insurmountable five or even ten percent Obama lead could vanish on election day. If Obama is to win, his campaign and supporters must keep the pedal to the metal to the last second, no matter what the polls say.

Chief among the Republican and racial biases is the Electoral College system. Another is the high possibility that a small but highly critical percentage of his white supporters may change their minds once they enter the voting booth, recoiling from electing a black president.

The Electoral College system greatly increases the possibility of a Republican victory.

First, that system gives as much as three times as much weight to the mainly conservative and white Republicans in the rural states compared with states with large, racially diverse and majority Democratic populations.

The rule is that each state is allotted the same number of electoral votes as they have congresspersons. Since each state has a minimum of one Senator and two Representatives, the states with the smallest populations have three votes.

For example, Wyoming has a little more than 200,000 voters and has three Electoral College votes: one for every 70,000 or so voters. By comparison large population states, like Florida, California and New York have about one Electoral College vote for every 200,000 voters.

Thus, the Electoral College system violates the principle of one person, one vote, and gives the Republican a major advantage in presidential contests.

Second, the Electoral College effectively nullifies the votes of nearly half of all black voters. Just over half of all black voters live in the Southern states and usually 80 to 90 percent of them vote Democratic.

Yet in 2000 and 2004, every single Southern electoral vote went to the Republicans. That is because whites are the vast majority of the electorate in every southern state, and about 70 percent of them vote Republican.

In both elections white Republicans out-voted black Democrats in every Southern state (and every border state except Maryland). Since Electoral College votes in each southern state go entirely to whichever candidate wins the plurality in each state, even if that plurality be just one vote, every single Electoral College vote from the South was awarded to Bush in 2000 and 2004.

The massive Southern black vote for the Democrats counted for absolutely nothing.

The pro-Republican biases of the Electoral College system were one of the reasons why George Bush was able to win the presidency in 2000 despite losing the national popular vote.

Barack Obama will have to pull out the stops to buck this system and win in November.

Obama will also have to contend with the so-called “Tom Bradley Effect.” This Effect refers to the documented tendency of a small but often decisive percentage of white voters who tell pollsters they are going to vote for a black candidate (or that they are undecided), and then vote for the white candidate.

This size of this Effect is hard to predict, but it could be a major factor in the dozen or so “battleground states” where the vote is closely contested.

In 1982, pre-election polls and even the election day exit polls gave California black gubernatorial candidate, Tom Bradley, such a sizeable lead that news organizations declared him the victor, virtually the moment the polls closed. Instead, he was upset by Republican George Deukmejian.

The same dynamic sunk Andrew Young’s gubernatorial bid in Georgia in 1990 and came within a hair’s breadth of erasing Douglas Wilder’s commanding pre-election lead in Virginia in 1989.

Barack Obama already had a brush with the Tom Bradley effect in the New Hampshire primary where his pre-primary lead vanished on election day. One can’t help but worry that the Effect will be even larger when the presidency, as opposed to a governorship, is at stake.

For example, if Obama goes into election day with a six point lead (say 53-47 percent), it would only take 3.01 percent of supposed Obama voters to change their minds to defeat him by 49.99 to 50.01 percent. Only five percent would need to change their minds to wipe out a ten percent lead.

So Obama supporters beware: there is no such thing as a sure thing lead for Barack Obama. No matter how far ahead he may get in the pre-election polls, we must fight to the end for every single vote.

BlackCommentator.com Guest Commentator, Marqueece Harris-Dawson, is executive director of the Community Coalition in South Los Angeles. Bob Wing is a writer who works with racial justice groups in Los Angeles. Click here to contact Harris-Dawson and Wing.