With two days left before Election Day, it remains clear that barring an historic upset, Barack Obama will be elected President of the United States on Tuesday.
The scale of an upset that would put John McCain in the White House is apparent by a look at the state-by-state polling numbers. According to Dave Leip's Atlas of Presidential Elections, McCain currently leads in 19 states with 160 electoral votes. There are five states - North Dakota, Missouri, Indiana, Ohio, and North Carolina - with 60 electoral votes that are considered to be tossups. There are another five states with 75 electoral votes where Barack Obama's lead is less than 10 percentage points - Nevada, Colorado, Pennsylvania, Virginia, and Florida.
In order for McCain to win on Tuesday, he would need to -
1) Hold on to all of the states where he currently leads, and
2) Win all of the tossup states, and
3) Win 50 out of the 75 electoral votes where Obama leads by less than 10 percent, or
4) Find some electoral votes in the states where Obama leads by more than 10 percent.
Assuming that #1 and #2 happen and that #4 does not, Florida - with 27 electoral votes - is an absolute must-win state for McCain. Pennsylvania - with 21 electoral votes - is an almost must-win. If Obama wins Pennsylvania, he would win the Electoral College with any one of Colorado, Virginia, or Florida. Obama and McCain would be tied in the Electoral College with Obama victories in Pennsylvania and Nevada (and McCain wins in the other three). Given the likely Democratic dominance of the next Congress, Obama would probably win an election in the House of Representatives.
If McCain wins Florida and Pennsylvania, he would still need to win one of Nevada, Colorado, and Virginia in order to win. (All the while winning all of the tossup states.) It is not inconceivable that this could happen, but it would very well fit the definition of the term "historic upset."
On Tuesday evening, the first state that might be projected is Virginia, where the polls close at 7PM EST. If Obama is projected to win there, it will be almost over for McCain. If Obama is then projected to win in Pennsylvania, where the polls close at 8PM EST, a McCain victory becomes virtually inconceivable.