SEATTLE, Wash. -- Billionaire real estate developer Martin Selig, a key financial supporter of Initiative 123, has switched sides.

The measure, which would preserve part of the Alaskan Way Viaduct and incorporate it into an elevated waterfront park, goes to voters August 2.

Selig contributed nearly $300,000 to the campaign, before putting $5,000 to the opposition in early June.

"I liked the idea that was first proposed," said Selig, "Now, the viaduct is all going to come down and I don't believe in that."

I-123's chief proponent is Kate Martin. Initially, Martin proposed retrofitting and saving the entire viaduct in a project similar to New York City's Hi-Line.

When that was deemed cost-prohibitive, Martin scaled the idea down to preserving part of the structure and building a new elevated park to replace the rest of the highway.

Her plan costs about $163 million.

As for Selig's defection, Martin is not angry.

"I've never met Martin Selig," said Martin. "He came to us and offered us money when we were looking for money.

"He left us, in debt, and promised not to support the 'No' side," she continued. "I'm not mad at him, I'd just love for him to pay us what he said he would."

The No on I-123 campaign called the switch "an unusual situation."

"What this says is that as people take a closer look at what I-123 actually says, and what it does, they're realizing it's a deeply flawed measure," said Sandeep Kaushik with the opposition.

Despite the setback, Martin remains confident in her campaign and her plan.

"They're hoping no one's going to notice that we're never going to have a world-class waterfront because it's all commercial buildings on the piers," she said. "(Our plan) has much more value and beauty and public benefit."