Republican Dino Rossi will announce Wednesday he plans to challenge Sen. Patty Murray (D-Wash.), according to media reports. 

A Washington State GOP source confirmed that a Wednesday announcement seems "likely."

Rossi ran for governor and lost twice to Gov. Christine Gregoire (D), falling short in his intial bid by 133 votes in 2004. He lost in 2008 by a more significant, 200,000-vote margin.

In April, Rossi told The Hill he regarded Washington, D.C., as a "snake pit."

"We have four children between nine and 19, so it's not a casual decision," he said. "I do have to say that I've been to Washington D.C., six times, and I'm not eager to head back into that snake pit. But I also see the flip-side of that, about what kind of a country we’re leaving to our kids."

Rossi has trailed closely behind Murray in some recent public polls and is perceived by Democrats to be a significant threat. The Democratic Senatorial Campaign Committee has spent recent weeks combing through his business records looking for ammunition.

Minnesota-based Republican consultant Pat Shortridge has signed on to become Rossi's campaign manager, according to reports.

Meanwhile, Murray has been busy preparing for a possible challenge. She raised some $1 million in the first quarter of 2010 and ended the month of March with $5.9 million in the bank, according to her Federal Election Commission filing.