When you boil it down, there are two types of elections: ones that are about continuity and ones about change.

For instance, we just had a change election in Pennsylvania.

Gov. Corbett was seeking a second term. His argument was that we should continue (there's that word) with the policies of his first four years. But, most voters were unhappy with Corbett's performance and they gave him a vote of no confidence, electing Democrat Tom Wolf. They wanted a change and they got it.

To bring it closer to home, the 1999 mayor's election was about continuity. The departing incumbent, Ed Rendell, was popular with the voters so the candidates running for mayor went to great lengths to assure voters they would continue his legacy, including John Street, who emphasized his work as Rendell's partner in governance.

Street, of course, turned out to be nothing like Rendell, and the 2007 mayoral election was about change. People didn't want another John Street in City Hall. Instead, they elected one of his severest critics: Michael Nutter.

What about 2015?

In my mind, this is a continuity election. Despite persistent problems (think: the public schools; poverty), things have gone well for the city in the last eight years: Crime is down; the population is up; the construction cranes for new buildings dot the skyline. We survived the Great Recession and the city enters 2015 on a roll, drawing the Pope this year and the Democratic National Convention in 2016.

Mayor Nutter can't take credit for it all, but Philadelphians give him credit for a lot.

Nutter has an approval rating of 57 percent among city residents, according to a poll paid for out of his campaign fund. That's high for an incumbent in his last year in office.

So, it struck me as strange that Nutter's name was never mentioned by the mayoral candidates at a joint appearance last week at a forum organized by The Next Great City, a coalition of reform and civic groups that trends green.

The six candidates present talked for two hours, but amidst their army of words deployed there were two --  "Mayor Nutter" -- that were never uttered. He was the Invisible Man, as far as the candidates were concerned.

It was especially strange given that the Next Great City coalition has done very, very well under the Nutter regime. In a card handed out to attendees (the room was nearly full despite miserable weather), the group listed six items on its agenda for this year. You can read them online at nextgreatcity.org.

I found the flip side of the card as interesting. The group listed the 10 items on their agenda in the 2007 election and five from 2011. They included such things as a new zoning code, the planting of more trees, a land bank, etc. All of them had check marks next to them -- meaning mission accomplished.

The man responsible for enacting most of these recommendations? Mr. Invisible himself.

It would have made sense for the candidates -- even just one of them -- to give a nod or even a verbal hug to the incumbent, thus giving those present a sense that as mayor he or she would (here's that word again) continue his "green" agenda.

Why didn't they?

I can think of three reasons, all equally plausible.

One. They don't know what they are doing. I don't mean the candidates are stupid, but at this early stage of the campaign they haven't honed their messages to be nuanced enough to give credit to Nutter where credit is due. Maybe this will improve with time. If not, this will be a campaign run in a fog.

Two. They prefer to emphasize style over issues. If you did a cloud of the word and its synonyms used most frequently, it would be "passion." It was mentioned more frequently than in your average Harlequin novel.

If you have a wonky, calm and cool incumbent, one way to differentiate is to characterize yourself as running hot, as a fighter for the city, as someone who cares deeply and (here's that word again) passionately about Philadelphia. Jim Kenney, Lynne Abraham and Nelson Diaz all burned hot. Doug Oliver was cool. Tony Williams was somnolent, but that may have been because he had spent 3 1/2 hours riding through a snow and ice storm to get from Harrisburg to the forum.

Three. As popular as Nutter is generally, he is despised by some powerful groups. As a candidate, you don't want to say a kind word about the mayor if you are speaking before the FOP, the firefighters union, the Philadelphia Federation of Teachers, and the city employees unions. They will toss you out of the hall.  To them, Nutter is anti-union and perhaps evil incarnate. Most of these candidates are wooing these unions for their support.

In each their own way, the candidates want this to be a change election, but all they are offering so far is a change in personality. And that's not enough.

This election is about continuity and the candidate who can best convince us they will continue the policies, the politics, and the advancement made in the last eight will have the best chance to win City Hall.