A survey last week from Public Policy Polling found Santorum had support from 38 percent of likely caucus participants, compared to 27 percent for Romney, 15 percent for Texas Rep. Ron Paul and 12 percent for former House Speaker Newt Gingrich.
However, half of the prospective voters said they could back someone else in Saturday's party caucuses.
Santorum's strong showing in the Washington poll parallels his surge nationally, coming off Feb. 7 wins in Colorado, Minnesota and Missouri and his conservative position on social issues such as same-sex marriage, coverage for artificial birth control among them, that have taken on a higher profile in the GOP presidential race.
Santorum was in Olympia the day after Gov. Chris Gregoire signed the state's same-sex marriage bill and met with leaders of the movement wanting to repeal it through referendum.
"That is something that Republicans in [Washington] state are very exercised about," Tom Jensen, PPP director of the polling, told The (Portland) Oregonian.
Jensen said about half of the surveyed voters who said they planned to attend the GOP caucuses identified themselves as evangelicals.
"You definitely have very conservative elements of the Republican Party coming out" for the caucuses, he said.
While he may have downplayed campaigning in Michigan and Arizona, which conduct their primaries on Tuesday, former House Speaker Newt Gingrich is making a "very diligent effort" to win the Washington caucus, his campaign spokesman told Fox News.
Washington's caucuses are non-binding, but there are 43 delegates at stake, the most since Florida (won by Mitt Romney) heading into Super Tuesday March 6, where Gingrich is focusing the lion's share of his stump time.
Washington would be a "pretty big score," Gingrich spokesman R.C. Hammond said.
"Washington state like many states in the West knows more than others how dramatic an impact the federal government can have on people's lives, whether it's the EPA [Environmental Protection Agency], whether it's the forest service, whether it's the Department of Agriculture," Hammond said. "There's no rancher, farmer or sportsman who doesn't know what it's like to have Uncle Sam breathing down your neck. So you're going to hear Gingrich talk a lot about how the federal government doesn't need to be an intrusive force in people's lives."
Republican presidential candidate Ron Paul, the libertarian U.S. representative from Texas has been traveling across the state, drawing large crowds in advance of the caucuses, The State Column reported.
"Thanks for inviting me to your revolution," Paul said during a recent appearance.
Santorum, the former U.S. senator from Pennsylvania, also led a poll available on the Facebook page of the Rev. Joe Fuiten, one of the leading Christian right leaders in Washington and senior pastor at the Cedar Park Assembly of God Church in Bothell, the Seattle Post-Intelligencer reported.
Early last week, Santorum had 81 votes, compared to Mitt Romney with 16, Gingrich at 13 votes and Paul with nine, the newspaper reported.