Here is why. The democrats actually do fairly decently with white people outside of the south. New England and northeast states are heavily white. So are states like Minnesota, Wisconsin, Washington and Oregon. But these states generally turn blue or have democratic senators/representativs.
Outside the south the dems do fine. However in the south the GOP wins the white vote about 65-90% of the time. So you average out the votes of Mississippi or Alabama, where whites are 90% GOP and you average it out with places like New England where whites are 40% GOP, and you average out to a GOP advantage. The deep south is about 80-90% GOP with whites. The rest of the south (Texas, NC, VA, etc) is closer to 60-70%.
But the point is the GOP doesn't win 55-60% of all white voters all over the country. They win white voters by huge margins in the south and rural parts of the rest of the country, lose white votes in the northern midwest/west coast and new england, and you average it out to a slight advantage for the nation as a whole. There are states where the GOP won the white vote in 2008, broke even, and lost the white vote. The white vote was about 70% democratic in Vermont, 52% in California, and about 10% in Alabama.
The problem is if you try and gain support from southern whites and rural whites from the non-south (Indiana, Idaho, Wyoming, etc) you have to change the party to do it. There is a reason southern whites and whites from rural areas in non-southern areas embrace the GOP. Because they harbor a lot of racial resentment and superiority towards blacks and latinos, they endorse religious fundamentalism, they resent the concept of community and being their brothers keeper, they disdain gays and are pro-militarism. There is a reason these people vote GOP, because the GOP shares their disdain for the weak and different. In order to win them over, the democratic party has to share that same value, which isn't going to happen.
As far as white people in general (being a white male myself), whites are a huge part of the democratic movement. Whites make up big majorities in liberals and union workers. Liberals are people who support left wing social & economic policy. They tend to be better educated and more involved in politics. They make up 19% of the electorate, and are about 80% white. Examples are Rachael Maddow, Olbermann, Howard Dean, Kucinich, Thom Hartmann, Sanders, Jon Stewart, Anthony Weiner, Bill Maher, much of DU, etc.
So the white people who I actually want to vote for and be part of the democratic party (white liberals and white union members) are pretty much on board already. The white people who I do not want to be part of the democratic party I don't want because then you have to fundamentally change the dem party to win them. The democratic party used to hold supermajorities among whites in the south until they stood up for civil rights. Then all those whites became GOP. So if you throw blacks, gays and latinos under the bus you might win a few white southern and rural votes. But its not worth it.
What really needs to be done IMO is improve turnout among people who support liberal/progressive concepts and democratic politicians, but who do not vote.
Single women now make up nearly 25% of the electorate, and they go dem 2-1. I've heard some people call single women the dem equivalent of evangelicals. Evangelicals make up 25% of the electorate and go GOP 2-1. However single women don't vote as often as married women (who are more GOP). Latinos and blacks do not vote as often as whites. Millennials do not vote as often as Boomers.
http://motherjones.com/mojo/2007/11/unmarried-women-are-democratic-partys-christian-evangelicalsSo the problem is turnout. The majority of the country supports democratic and liberal policy. But voter turnout is skewed to benefit the GOP. The more demographics favor the GOP, the higher turnout goes.
Whites have higher turnout than blacks/latinos
Boomers, Xers have higher turnout than millennials
Married people have higher turnout than singles
Higher income people have higher turnout than lower income people
So if you address the issue of voter apathy, then you can gain several million dem votes w/o changing the identity of the party. I've even heard it be said that if people made a meaningful voter registration drive effort in Texas that Texas would go blue.
In fact if you address the discrepancy in income alone Carter would've won in 1980 and Gore in 2000.
http://www.dollarsandsense.org/archives/2004/0104cervantes.html