Mayor R.T. Rybak has given the city of Minneapolis 11 years of vigorous, visionary leadership and the promise of just one year more. He announced on Dec. 28 that he won't seek re-election.

That means he's also giving his city an opportunity for something that might not have happened had he opted for a fourth term: There's a chance that in the next 10 months a lively mayoral contest will ensue, one that engages a large share of the city's voters and builds a mandate on which the next mayor can stand and the city can grow.

Rybak's achievements and popularity made the 2009 mayoral campaign a yawner. It did little to advance citizen understanding of the challenges the city faces in the next decade as it confronts shrinking federal and state aid and rapid demographic change. A citywide discussion is in order about how best to grow the property tax base, maintain public safety, renew aging infrastructure and make the most of an aging and increasingly diverse population.

Rybak's retirement does not ensure such an election in 2013. Neither does the colorful cast of political characters who are either officially in or are known to be considering mayoral bids.

For Minneapolis to have the constructive, forward-looking campaign it deserves, the city's dominant DFL Party also must do its part. It should revise its endorsement practices to avoid short-circuiting the mayoral race before it has begun for rank-and-file voters.

The party should create conditions that allow several strong candidates to take their bids to the Nov. 5 ballot. At a minimum, delegates could refrain from insisting that contenders for endorsement promise to end their campaigns if they are losers at the June 15 convention.

Alternatively, DFL candidates for mayor could agree, with at least tacit support from party officials, that they will make no pledge to "abide" if their convention bids come up short.

Or the party could do as a blue-ribbon state election reform commission recommended 15 years ago: It could bestow multiple endorsements, allowing up to three candidates to proceed to the ballot with official DFL blessing. In the spirit of the city's ranked-choice voting system, the party could rank candidates and share its campaign resources proportionately among them.

Some party insiders are bound to respond to these suggestions by saying a large mayoral field diminishes the chance that convention delegates will reach the 60 percent consensus that endorsement requires.

But party rulemakers should consider whether delegates even ought to try. Ranked-choice voting arrived in Minneapolis in 2009 with DFL backing and a promise that it would give voters a chance to evaluate and rank multiple candidates on the general-election ballot. No prior primary would winnow the list. Voters would be free to reward with second- and third-choice votes those candidates who demonstrate broad appeal, thereby helping one of them achieve victory with a majority, not just a plurality, of votes.

That promise will be nullified if the DFL convention succeeds in bestowing its customary solo endorsement accompanied by its customary insistence that losers drop out.

In fact, it might not take much insistence to shrink the field. With no primary in August, a lone DFL endorsee will have nearly five months, from June 15 to Nov. 5, to take advantage of the DFL's considerable resources in the nearly one-party town. That advantage stands to discourage all but the most motivated and best-funded challenger.

Party insiders aren't likely to warm to the notion that they should surrender or share their power to be mayor-makers. They would do well to consider Rybak's recent reflection on his first campaign, in 2001.

A political newcomer, Rybak succeeded that May in blocking the endorsement of then-Mayor Sharon Sayles Belton, then spent the summer "knocking on tens of thousands of doors." The contacts he made were invaluable not only to his election, but also to governing, he said.

"I learned a lot in the endorsement process. I learned a lot more that summer. A mayor needs to know a lot of people, not just those who participate in the endorsement process. We ought to give people time to know the candidates, and give the candidates time to grow."

The timing of Rybak's retirement announcement affords 10 full months for candidates and citizens to interact. The DFL Party should allow them to make the most of that time.