Editor's note: This is the fourth in a series of questions The Next Mayor is asking — and answering — of the current candidates' campaigns. Monday: Lynne Abraham's campaign. Tuesday: Nelson Diaz's campaign. Wednesday: Doug Oliver's campaign. Tomorrow: James Kenney's campaign.

Question 4: Because of the habit of black voters supporting black candidates, doesn't Sen. Anthony Williams have a lock on the Democratic nomination for mayor?

You might want to ask Lynn Swann about the habits of black voters in Philadelphia. In 2006, Swann -- a former Pittsburgh Steelers star and an African American -- ran for governor as a Republican and tanked in Philly.

He got the lowest percentage of votes -- in black and white wards alike -- of any recent Republican, including the much-disliked Tom Corbett.

Swann was running against Gov. Ed Rendell, the former mayor, who was immensely popular in his home base, and that certainly was a factor.

But, you can chalk up Swann's poor showing to his party trumping his race among the city's black voters. They have become increasingly Democratic (that's with a big D) in recent decades.

In the 1970s and 80s, it was not uncommon for a Republican candidate to get double-digit support in black wards. In 1978, for instance, white Republican Dick Thornburgh got 48 percent of the black vote. Arlen Specter polled 30 percent or better among the city's black voters whenever he ran for office (which was a lot).

Today, that would be unimaginable.  True, Barack Obama won 99 percent of the black vote in 2012, but that's only a tiny step better than the very white Al Gore and John Kerry, Democratic candidates who got 97 percent and 96 percent of the black vote respectively.

In primary elections, where all the candidates are reliably Democratic, black voters do favor black candidates. (And, no surprise, white voters favor white candidates.)

This may be post-racial America, but racial and ethnic ties still run deep.

The math this year is fairly straightforward: If the past is any guide, 55 percent of the Democratic voters who show up to vote in the primary will be African American. If Sen. Anthony Williams gets 90 percent of the black vote, it will add 136,000 to his tally -- enough to win the nomination for mayor.

There are several "ifs" in this equation. While Williams is the most formidable black candidate in the race, there are three other African Americans: Milton Street, Doug Oliver and the Rev. Keith Goodman, a North Philadelphia minister who is a late add-on to the field.

If the three combine to get 20 percent to 25 percent of the black vote, that will put a big dent in Williams' share of the black vote.

The white candidates in the race for mayor -- Lynne Abraham and Jim Kenney -- have run well among African American voters before and will undoubtedly go for a slice of that vote this year.

In 2007, for instance, we had a field of formidable African Americans -- Michael Nutter, Dwight Evans and Chaka Fattah -- all running for mayor. Yet, the white millionaire Tom Knox got a 15 percent share of the black vote.

There's another factor other than race that may make a difference: geography.

Williams' power base is West Philadelphia. His Eighth Senate District is centered there -- though it does sprawl into both South Philly and Upper Darby. He is leader of the Third Ward, which encompasses the Cobbs Creek section of the city.

What is missing this year is that there is no candidate -- at least not so far -- from Northwest Philadelphia. The Northwest is home to a number of powerhouse wards -- the 10th and 50th are two -- that are majority black.

For years, West Philly and the Northwest have had competing black political organizations. Rival clans, if you will.

Will the rivalries of the past influence the competition in the May primary? For instance, can Williams make peace with and get the full support of Northwest Democratic political leaders? Can Jim Kenney -- a white guy in the race -- get support from former Council colleagues Marion Tasco (leader of the 50th Ward) or Council President Darrell Clarke, whose district encompasses North Philadelphia? Will Lynne Abraham snatch a portion of the vote among elderly blacks? That's why we have campaigns -- to answer questions like these.

Bottom Line: Williams starts with the largest potential base, but he won't get it without a fight.

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.