Editor's note: This is the opening piece in a week-long look at the landscape of Philadelphia's voting trends and what it means to the candidates' chances in the May primary. Also today: Debunking the "too old" logic surrounding Lynne Abraham. Coming tomorrow: Critical look at Latino voting trends and their effect on the hopes of Nelson Diaz.

The magic number is 100,000 votes.

Recent history tells us that is the total a mayoral candidate will need to win the Democratic nomination in the May 19th primary.

In 1999, John Street won in a six-person field by getting about 101,000 votes -- 35 percent of the 285,000 votes cast. In 2007, Michael Nutter won in a five-man field by getting 107,000 votes -- 37 percent of the 290,000 votes cast in that primary.

It may seem odd that in a city of 1.5 million, a mere 100,000 voters can determine who will run the city for the next four to eight years.

But, that is the way it is. Voter participation in this city is low and getting lower, according to City Commissioner Al Schmidt, who has studied turnout history.

How can we have nearly as many voters as adults in the city (1,036,000 registered voters out of 1,191,000 adults over 18)? With such a huge pool of potential voters, how can the turnout numbers be so low? Why so many ghosts in the vast one-million-plus voter list?

It was a mystery that cried out to be solved.

The answer was found in a large database provided by City Commissioner Al Schmidt, who helps oversee elections, filled with information on voters and their habits: name, age, sex, ward, division, streets address, plus a record of how they voted in the last 10 elections — five primaries and five general elections held since 2010.

Dive deeper and you can start to separate voters into two broad categories: active and inactive.

We set a low bar for active: anyone who voted at least once in the 10 elections held since 2010.

Inactive was anyone who either had not bothered to register or had not show up to vote for more than five years.

Once we sorted those hundreds of thousands of voters into the two bins, a clearer picture emerged.

Of the 1.2 million adults 18 and over, more than 455,000 are inactive. That's equal to 38 percent of adults. Again, they are inactive because they have either never registered to vote or, if registered, haven't voter in more than five years.

That leaves a core of 739,000 active voters. Most of them (about 600,000) are Democrats. The rest are Republicans, independents or members of minor parties.

There is a large gap between the official number of registered voters and the number of active voters. On the books, there are 1,036,000 Philadelphians registered to vote, but nearly 30 percent of those registered simply haven't voted. They are the ghosts in the machinery.

Why do they remain on the rolls? Because federal law makes it difficult to purge voters. The Election Bureau can't begin the process of removing them until they have missed voting for five years. Then, if they fail to show up for the next national election, they can be purged. The next national election is the 2016 presidential race.

In other words, it can take up to seven years to purge a voter who may have moved or simply opted out of voting. (Those who die have their names removed more quickly because the state shares information on death certificates with local election boards.)

Let's focus on the Democrats. There are 807,000 registered, but 207,000 of those are inactive.

That leaves 600,000 active Democrats, though some barely so. Nearly 30 percent have voted only once in the last 10 elections.

The presidential election of 2012, when Barack Obama was running for re-election, was the high point for active Democrats: 92 percent of them came out to vote that November.

Not all voters are created equal. About 76,000 active Democrats (about 13 percent) are Super Voters.  They have voted in at least eight of the last 10 elections.

Not all wards are created equal either. There are about a dozen Power Wards where voter participation is much higher than the city average. The most notable are the Ninth Ward (Chestnut Hill/Mount Airy); the 50th Ward (Cedarbrook/Stenton) and the 10th Ward (West Oak Lane). They are all in the city's Northwest.

Then, there are wards that are the equivalent of 90-pound weaklings, with anemic voter participation. Most of these 16 wards are located in the city's poor neighborhoods.

It's well known that voters over the age of 65 tend to be more active than younger ones. The surprise is the difference. Older voters have more than double the participation rate of those aged 18 to 34 in the city. Millennials may be the future of the city, but senior voters are a powerful present.

As a Democrat candidate for citywide office, it can make you dizzy to think you have to appeal to 600,000 active voters. In reality, you don't. For starters, if the past is a guide, more than half of them won't show up to vote on May 19th.

What you need to run for mayor is a base: a starting point of strength with appeal based on locale, race, ethnic origin, public record, age or all of the above. The stronger your base, the easier it is to reach the magic 100,000 mark.

For instance, Doug Oliver, at age 40 the youngest of the six candidates, is making an overt appeal to young voters. Not a bad strategy, given that there are 165,000 active Democratic voters aged 18 to 34 in the city. If he can get 60,000 voters from that age group, he will need just 40,000 from everyone else. Easier said than done.

About 90 percent of politics is BS and 10 percent is cold, hard numbers.

The hardest are the actual results on Election Day. Money raised is another. Opinion poll numbers can be sometimes misleading, but they help, too. Now, we have hard numbers active Democrats -- the only ones who really count.

Over the next five days, we'll look at questions of voter base as it applies to each of the major candidates. We can't predict the outcome of the race -- that's why they wage  campaigns -- but we can attach numbers to candidates as a beginning point.

It will change your perceptions of the race for mayor.

Tom Ferrick Jr., a former Inquirer reporter and columnist and the founder of the news site Metropolis, is writing regularly on the 2015 mayoral race for The Next Mayor.