Tuesday night's first Democratic debate made clear a fascinating contrast that's emerged in this presidential election: The Democratic and Republican fields aren't simply offering different visions for America's most urgent policy challenges — they're often talking about different challenges all together.

Republicans are running in a world where border security is central and threats to religious liberty and gun rights run high. Democrats, on the other hand, see a country where wealth inequality looms large and climate change is rapidly approaching. Republicans are worried about the sanctity of life and the consequences of vaccines. Democrats, who have said hardly anything on those topics, dwell instead on paid leave and Wall Street.

Of course, there are plenty of policy areas where the two have overlapped (even if their positions haven't). Both fields have talked in their debates about gay marriage, marijuana and Social Security. But the many areas in which they're not even engaged in the same conversation are striking:

That diagram covers policy topics that have received more than a passing reference in the three debates so far (sorry, we're not including the Republican under-card debates). To be fair, the Republicans have had two debates and the Democrats only one. But substantial time in both GOP debates has been devoted to questions about character and temperament — and things Donald Trump has said — that had little to do with specific policy topics. As for which field has covered more issues, the score is about even.

Debate topics are also admittedly chosen by moderators and not the candidates themselves, but the questioning has mirrored many issues on the campaign trail, and we've included topics candidates raised in their opening and closing remarks.

Both groups have spent substantial time talking about Russia, Syria, Iran, Iraq, Afghanistan, Libya and nuclear weapons, so we've grouped all of those topics into "foreign policy" above. The other issues in the middle of our Venn diagram have been touched on by both parties, but not necessarily with equal heft (the "Black Lives Matter" talk in the first GOP debate, for example, was fleeting).

The areas where the two don't overlap, though, are in some ways the most interesting, as they reveal two presidential fields that often sound as if they're running in completely different elections. As for the topics left out by either party at the bottom, we're not giving anyone credit for cursory mentions of the poor.