Former King County Executive Ron Sims will not join the crowded field of challengers to Seattle Mayor Mike McGinn this year.

Ending months of speculation, Sims went on KUOW’s Weekday radio program Monday to announce his intentions.

For much of the 20-minute interview, Sims sounded like he was ready to announce a candidacy. He said Seattle needs “some vision,” touted his own record and made an impassioned case that the next mayor should wield a bully pulpit to demand “world-class schools.”

But in the end, Sims bowed out, saying “There are other mountains I want to climb … I am not going to run for mayor.”

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Sims’ decision to stay out of the 2013 contest was a relief to some others in the race.

State Sen. Ed Murray beamed Monday in the Senate wings as he relayed the news to a reporter. Murray announced his exploratory bid for mayor in December.

A poll for KING 5 last week found Sims, even with no announced campaign, was tied with McGinn at 15 percent support among registered voters — a higher number than for any other challenger.

“I’m convinced he (Sims) would have won,” said Tim Ceis, a former Seattle deputy mayor and Sims’ former chief of staff.

The Sims news could also benefit McGinn, as the KING poll suggested he’d pick up the greatest share of Sims supporters.

But the overall numbers don’t look great for the mayor: His job-approval rating stood at 37 percent in the poll, compared with 43 percent disapproval (19 percent were unsure).

John Wyble, a McGinn campaign consultant, conceded the polling didn’t look “great.” But he said, “We’re in the game” to get through the August primary.

Even without Sims in the race, McGinn already faces seven announced challengers.

Besides Murray, they include City Councilmembers Tim Burgess and Bruce Harrell and former Councilmember Peter Steinbrueck.

As for what his future holds, Sims told KUOW “maybe it’s some crazy village on a mountain” or digging “wells for clean water or disease control.”

One of Seattle’s best-known politicians, Sims served a decade on the Metropolitan King County Council before being appointed King County executive in 1996.

Sims won election to a full term as executive the next year and was re-elected in 2001 and 2005. He sought higher office twice, running unsuccessfully for the U.S. Senate and for governor.

In 2009, Sims was appointed by President Obama as deputy secretary of the U.S. Department of Housing and Urban Development. He resigned from that job in 2011, citing the difficulty of spending so much time away from his family.

Seattle Times staff reporter Andrew Garber contributed to this report. Jim Brunner: 206-515-5628 or jbrunner@seattletimes.com. On Twitter @Jim_Brunner