When the U.S. Treasury Department announced that they were bumping Andrew Jackson off the front of the $20 bill to be replaced by Harriet Tubman, most conservatives and Republicans praised the decision. But not Donald Trump.

Instead, Trump derided the decision as " pure political correctness" and suggested maybe Tubman appear on the little-used $2 bill instead. Trump's comments followed those of Fox News' Greta Van Susteren, who charged that the Obama administration " went stupid" with the decision to bump Jackson, supposedly picking a "completely unnecessary fight" that was "dividing the country."

In reality, Jackson is a divisive figure, having expanded executive power and forcibly removed Indian tribes from their native lands. Meanwhile, it's hard to think of a less controversial figure in U.S. history than Tubman, the gun-toting escaped slave who ran daring missions to escort 70 slaves to freedom.

Typically, I'd dismiss Trump as an outlier for his comments, but it's harder to do that given that he's the Republican front-runner who has won more votes than any other candidate. The episode illustrates some of the broader challenges facing Republicans seeking to win over black voters.

There's a widely held assumption, backed up by decades of election data, that black Americans are always going to vote nearly exclusively for Democrats.

It's true that ideologically, black voters tend to be more liberal and more supportive of a larger role for the federal government. Much of this comes from historical experience. Conservative arguments in favor of giving more control to states are tainted because in the past similar sounding arguments were used by those who wanted preserve slavery and segregation. And today, many black Americans see benefits in government programs that conservatives are eager to cut. That having been said, polls suggest that black voters should be more amenable to the conservative message than their voting patterns would indicate.

The most fundamental ideological question in American politics involves the debate over the proper role for the federal government. Interestingly, a Pew Research Center report released last year found 32 percent of black Americans would prefer a smaller government with fewer services to a larger government with more services. Though that's less than a third, consider that in elections dating back to 1980, Republicans have averaged just under 10 percent of the black vote, and never more than 14 percent (which was in 1980).

Another extensive poll taken in 2012 by the Public Religion Research Institute also provides evidence that ideologically speaking, black voters are not as uniformly liberal as one would assume looking purely at their voting history. For instance, 48 percent viewed the federal deficit as a "critical issue," slightly edging out income inequality; 52 percent saw religious liberty as being under threat; 30 percent said abortion should be illegal in all or most cases; 51 percent said abortion was morally wrong; and 35 percent said religiously-affiliated colleges and hospitals should not be required to provide employees with free contraception coverage.

Does this mean that Republicans should be able to convert all of those voters?

Of course not. But they wouldn't need to. Just take the 2012 election as an example. FiveThirtyEight has a useful interactive tool that enables you to adjust the vote percentage among several key demographic groups to see how changes could alter the electoral map. For instance, if Republicans were to hold Democrats to 82 percent of the black vote and all else where equal, Ohio and Florida would shift to Republican hands along with their 47 electoral votes.

If resistance to Republicans among the black community cannot be explained by ideology alone, then, what else can it be attributed to?

Another aspect is that for all the official efforts at black outreach among national Republicans, and attempts at racial sensitivity by elected officials, whenever the issue of race is in the news, there's always a Republican or conservative media figure somewhere saying something off-putting.

What gets communicated to blacks is that a lot of Republicans are resentful toward them and dismissive of any complaints about modern day racism. When the Republican front-runner, instead of using the Harriet Tubman news as an opportunity to celebrate an American icon, takes the chance to slam "political correctness," it's one other incident that reinforces this impression.