Seattle Mayor Ed Murray will not run for re-election as a write-in candidate, a source close to Murray's inner circle said Thursday. A formal announcement will be made Thursday morning.

Murray would not indicate on Wednesday which way he was leaning. He commissioned a poll last week to gauge the viability of a write-in campaign.

If he had re-entered the race, Murray would have to file as a write-in candidate and commissioned a poll last week to assess support for such a campaign.

“You know write-ins are weird,” said Murray Wednesday. “The polling I've done shows there's a path forward, but write-ins have real challenges and our state laws have particular challenges when it comes to write-ins. Hopefully, in the next couple days.”

If Murray had filed as a declared write-in candidate, his name would not have appeared on the ballot, but any variation of his name or misspelling would potentially count, according to the King County Elections Office.

However, using a tool such as stickers printed with the write-in candidates’ name would not be allowed, under state law.

Also, a write-in candidate’s total number of votes would not be checked and officially counted, unless the number reached a certain threshold—greater than or closer to the number of votes cast for the second-place candidate.

Political strategists say write-in campaigns take extra effort and organizing.

The most high-profile, successful write-in campaign in Washington is probably a 1994 congressional race in southwest Washington in which Republican Linda Smith defeated the Democratic incumbent, according to the Secretary of State Office.

In the city of Seattle, the archives department points to a 1990 election involving a Seattle Municipal Court judge who ran and won a write-in campaign.

However, an additional challenge for Mayor Murray comes from the fact that key parts of his campaign infrastructure and support have since moved on to other campaigns, including those of former U.S. Attorney Jenny Durkan and former State Representative Jessyn Farrell.

His endorsements have also begun to scatter, and his campaign fund has under $100,000, according to the latest filing. Some of that money has likely been used for the mayor’s most recent poll.

Jenny Durkan now leads in the fundraising numbers with more than $286,000. She’s also amassed support from the Seattle business community and the sole endorsement of the Seattle Metropolitan Chamber of Commerce’s political action committee, CASE.

A look at contributions to date by mayoral candidates

Meanwhile, endorsements from key labor groups and state legislators have begun moving to both Jessyn Farrell and State Senator Bob Hasegawa’s campaigns.

One of the region’s most powerful unions, SEIU 775, which represents health care workers, has not yet endorsed a new candidate.

“We never retracted our endorsement,” President David Rolf told KING 5 on Wednesday.

When asked whether Rolf would have supported the mayor, Rolf said: “He’ll have our support for whatever he wants to do next.”

However, whether there would be a dual endorsement by SEIU, if Murray got back in the race, remains to be seen, added Rolf.

The mayor would not have had a lot of time to re-launch a write-in campaign, with the August 1 Primary just weeks away.

It’s also too early to know whether the majority of his former supporters and donors would have returned to his side.

KING 5 reached out to a variety of political insiders and players on Wednesday, and it appears Murray’s supporters are split; some have indicated they’ve moved on to other candidates.

However, former council member John Okamoto said his support for the mayor remains.

“Ed should give voters a chance to decide whether his record of accomplishments deserves another term,” said Okamoto.

In May, Murray announced he was withdrawing his re-election bid, facing a civil suit alleging sexual abuse in the 1980s.

Murray has denied all allegations and has said he believes the suit was politically motivated.

Earlier this month, the mayor’s accuser dropped the suit, saying he wants to wait until Murray is out of office to pursue the case.

"Delvonn (Heckard) doesn't care about the mayoral race, and never did. The Mayor's decision not to run just helped confirm to Delvonn he was heard,” said attorney Lincoln Beauregard in a statement to KING 5.

“However, Delvonn is angered by the Mayor's claims of vindication, and personal attacks on his lawyers, which could cause him to want to move forward sooner -- let's hope that is not necessary,” Beauregard continued.