Hillary Rodham Clinton spoke at a gathering of female leaders in Silicon Valley on Tuesday, so it's probably not terribly surprising that she hit gender themes pretty hard. But we can fully expect that she will keep reinforcing those themes as her inevitable presidential campaign gears up for a very simple reason. Clinton does well with women -- and there are a lot more women than men in the United States.

About 5 million more, according to the Census Bureau's 2013 estimates, across all age groups. In our look at Clinton's California visit, we noted that current polling shows her doing several points better among women, particularly older ones. But those extra 5 million women don't all vote (and aren't all registered, as the number includes people younger than 18), and are scattered across the country.

If you look at the ratio of men to women by county, it's all over the place.

But if you look at a subset of that -- the percentage of men older than 50 vs. women older than 50 in each county -- you start to see something interesting.

In two-thirds of the nation's counties, there are more women than men, but the overall average difference in the percent of men vs. the percent of women is zero. But in all but 41 of the country's 3,143 counties, there are more women than men older than 50 -- by a margin of three percentage points.

That doesn't mean much if there aren't many people older than 50 in the county. After all, having a 9-to-1 ratio of women older than 50 to men older than 50 in a city where there are only 20 people older than 50 doesn't add up to many votes. But the number of women older than 50 is huge. In 2013, nearly one in five people in the United States was a woman older than 50. In many counties, women older than 50 make up at least a fifth of the population.

It barely needs to be said, but we'll say it anyway: Older Americans are more likely to vote.

Having women older than 50 on your side is a strong political position, but it's hardly a guarantee of victory. People usually pick candidates based on their political philosophies, not on their demographics, and a lot of those women older than 50 are Republican. But if Clinton continues to poll well with her gender, which seems likely, you can expect a lot more events that have a theme similar to Tuesday's.