I ended last week with an appearance on Hardball, with guest host Chuck Todd, discussing this Ron Brownstein piece on “the changing American electorate” in National Journal.

The general gist of the article is that the rapid growth in America’s minority populations — most notably Hispanics and Asian-Americans — is bad news for the GOP. By their calculations, Obama can do much worse among white Americans in certain key states and still win, because the growth in the share of the electorate by these other groups would counteract that.

I might argue the reverse, that some polling evidence suggests Obama’s support among whites is eroding, and he’ll need greater turnout among minority voters to offset those lost votes.

Obama won 43 percent of the white vote in 2008, and in a lot of the key swing states, Obama was winning percentages ranging from the mid-40s to more than 50 percent.

Obviously, Obama’s current approval rating doesn’t correlate perfectly with his likely share of the vote, but let’s presume that his share of the vote is within a few points of his approval rating. A recent National Journal poll finds, “Obama’s approval rating among whites remained at just 39 percent; it hasn’t cracked 40 percent since September 2009.” Even worse for the president, Quinnipiac’s latest puts Obama’s job approval among whites at 29 percent.

Obama won 43 percent of the white vote in 2008. When National Journal was trying to calculate how little of the white vote Obama could get and still win these states, the lowest swing states were North Carolina and Virginia at 38 percent. But if the publication’s own poll is accurate, Obama is really on the precipice of how little of the white voter support he can enjoy while remaining competitive, and if the Quinnipiac poll is accurate, he’s up a certain creek. (PPP puts his approval rating among whites at 35 percent in North Carolina and 37 percent in Virginia; recall National Journal calculates Obama needs 38 percent with higher minority turnout.)

Among Hispanics, Obama is doing much better, with a 64 percent approval rating according to Pew. In Gallup’s poll, it’s been as low as 54 percent. The organization Latino Decisions did a poll in February that put Obama’s approval rating pretty high — 70 percent — but only 43 percent were certain to vote for him. Obama won 67 percent of the Hispanic vote in 2008. If Obama wins a percentage close to those 70 percent who currently approve, he’s home free. If Obama’s level of support is closer to that 43 percent of current certain supporters, he’s in deep trouble.

Of course, one problem with this type of analysis is that we’re looking the numbers in a vacuum, presuming that past voting habits continue. For example, the unemployment rate for blacks is currently 13.7 percent and the unemployment rate among Hispanics is 11.3 percent. For almost all of Obama’s presidency, these troubling numbers have been even higher. Will Obama run as well among those groups if unemployment among them is significantly higher than it was in 2008?

Finally, I would note that in every election, and certainly since 2008, Democrats have counted on minority voters to turn out and support them in large numbers. By and large they have been disappointed, even in states with high Hispanic populations like New Jersey, Florida, New Mexico, Arizona, Massachusetts, and Virginia. Will turnout be up in a presidential year? Sure. But it’s worth noting that in the past two years, some very well-funded campaigns brought President Obama to rallies in places like Cleveland and Norfolk and Newark and Camden, and minority voters haven’t shown up for the Democrats in the numbers they were looking for. Sometimes, the voters just don’t show up.

UPDATE: Some more thoughts on this issue from Michael Barone here.