"To a significant extent, the Republican base does have elements that are animated by racism. And that's unfortunate," Democratic Congressional Campaign Committee Chairman Steve Israel (D-N.Y.) said in an interview broadcast Sunday on CNN.

Israel's GOP counterpart Greg Walden (R-Ore.), also on the show, rejected the idea that race plays any role in GOP opposition to Obama's policies.

Racial bias is incredibly difficult to quantify. You can't simply ask people if they're racist because nearly everyone -- racist or not -- will say no. That's partly because prejudice often acts at the subconscious level (Want proof of this? Go take the implicit bias test developed by researchers at Harvard, the University of Washington, and the University of Virginia).

So pollsters often try to measure prejudice indirectly. An Associated Press poll conducted in 2012 attempted to measure implicit racism among Democrats and Republicans by asking respondents to compare black, white, Asian and Hispanic faces. It found that 55 percent of Democrats expressed implicit anti-black attitudes, compared with 64 percent of Republicans -- a difference that the lead researchers called "highly significant."

Another proxy for measuring racial prejudice is a person's views on interracial dating and marriage. The Pew Research Center has been polling on this question for 25 years. In 2012, 18 percent of Republicans disapproved of blacks and whites dating each other, compared with 5 percent of Democrats.

Perhaps even more telling is that this gap has widened in recent years. In 1997, nearly identical numbers of Democrats and Republicans disapproved of interracial dating, at 30 and 31 percent, respectively. Democratic disapproval of interracial dating has softened considerably faster than Republican disapproval since then.

So, taken at face value, Israel's statement does have some support: Data suggest that a section of Americans who identify as Republicans express measurable racial prejudice at a higher rate than their Democratic counterparts.

This doesn't mean that all Republicans are racist, of course, or that all Democrats are free of racism. Indeed, as the Harvard implicit bias test shows, we're all carrying around racial baggage of some sort or another. But the survey findings above indicate that Republicans have more work to do to address such prejudice within their ranks. There's an obvious moral imperative behind this, but there's also a strategic one: A party can't sustain itself if it continues to grow older and whiter while the rest of the nation grows younger and more diverse.