Democrat Nicholas Mattiello said he won the support of 41 legislators in a vote taken during a hastily-called caucus meeting, three more than is needed to assume the speakership. He said that includes 39 Democrats and two Republicans.
The Sunday night caucus comes a day after Fox relinquished his leadership post following raids at his Statehouse office and home as part of a criminal investigation. Officials will not say whom or what they are investigating.
Mattiello, who served as Fox's top deputy, said through a spokesman later Sunday that he had picked up five more supporters ahead of a formal vote expected on Tuesday. The speaker must be elected in an open House session.
"I'm not the speaker yet," Mattiello said. But, if elected, he added, "My biggest concern is uniting and unifying the House."
Mattiello said his support is not the product of any deals cut or conditions set. He indicated he would examine all aspects of the House and how it operates and bring his own style to the body. He described himself as "pro-business" and said that legislators must work to move the economy forward.
House Oversight Chairman Michael Marcello, who is challenging Mattiello for speaker, met earlier in the day in Johnston with his Democratic supporters as well as five of the six Republican members of the 75-member chamber, whose backing he is seeking.
Marcello told reporters outside the meeting he had 33 votes, five votes shy of what's needed. Asked about his claim late Saturday that he had enough votes, he said, "Everything's still in play."
"We're going to take it all the way to the House floor," he said.
Marcello said he represents a "sea change" in the way the House would be run and that he wants a more open legislative process.
Rep. Joseph Trillo, the House minority whip, went to both meetings. Earlier in the day, he reported that he was leaning toward Marcello. He said Republicans want to vote as a bloc but that he didn't think they were "100 percent either way."
After the caucus Sunday night, Trillo emerged saying he would vote for Mattiello. He described himself as torn but said Mattiello has shown "more strength."
The Friday raids on Fox's office and home came amid a joint investigation by the U.S. attorney's office, FBI, IRS and state police.
Fox, a 52-year-old Providence Democrat who became the nation's first openly gay House speaker in 2010, has not addressed whether he is the target of the investigation, what authorities are probing or even whether he has hired a lawyer.
In a statement Saturday, he simply said he was stepping down. "Because of the respect I have for all members of the House of Representatives, I am resigning as speaker," he said. "The process of governing must continue and the transition of leadership must be conducted in an orderly manner."
He said he will serve out the rest of his term, which runs through the end of the year. "My personal focus going forward will be on my family and dealing with the investigation," he said.
Fox's resignation—which, for all practical purposes, was effective immediately—is expected to be read into the House record Tuesday. He plans to submit a letter of resignation to the secretary of state's office as a formality, though a spokesman for that office said that doesn't appear necessary.
Fox has represented Rhode Island's capital in the General Assembly for more than 20 years and is one of the state's most powerful politicians. His enduring legislative legacy is most likely to be legalizing gay marriage. In 2011, he abandoned an effort to do so because of opposition in the Senate. He shepherded through civil unions instead and was roundly criticized by some gay marriage supporters at the time.
In 2013, Fox was instrumental in pushing gay marriage legislation through as the political climate shifted nationally.
Associated Press writer Michelle R. Smith contributed to this report.