Encouraged by the feedback he's received, Sen. Bruce Tarr (R-Gloucester) is still contemplating a U.S. Senate special election campaign and discussing whether he believes he could be more productive in Washington or as state Senate minority leader.

Tarr planned to decide by Monday whether to run for the seat given up by Secretary of State John Kerry, but said his decision-making process was extended because of time he devoted to storm response in his Senate district and passage of a $115 million spending bill in the Senate on Tuesday.

In a late-morning impromptu interview with the News Service and a Boston Globe reporter, Tarr indicated he'll make his decision by Friday, if not sooner.

Tarr described himself as "a bit overwhelmed" by the response to his potential candidacy. "It's been very encouraging,” he said.

Tarr also acknowledged former U.S. Attorney Michael Sullivan may join a Republican primary field that currently features businessman Gabriel Gomez of Cohasset and Rep. Dan Winslow of Norfolk. A state representative from Abington from 1991 until 1995, Sullivan served as Plymouth County District Attorney from 1995 until 2001. Currently a partner at Ashcroft Law Firm, Sullivan before his public service career practiced law and worked for 16 years at Gillette, where he started out as a stock clerk.

Sullivan has not returned calls from the News Service.

The size of the field is "not really" a consideration in Tarr's decision-making, said Tarr, who heads up a four-man GOP caucus in the 40-person state Senate.

"I still think this is a wide open situation," Tarr said.

While Republicans were slow to enter the race after Reps. Edward Markey and Stephen Lynch declared their candidacies on the Democratic side, Tarr said the idea that the GOP wouldn't be able to come up with candidates is proving wrong. "We're not going to have that problem at all," he said.

All of the declared and potential candidates face the significant task of gathering 10,000 valid nomination signatures by Feb. 27. If he decides to run for Senate, Tarr plans to use both volunteers and paid signature-gatherers, he said.

Tarr said he had talked with former Sen. Scott Brown about running and said he believed Brown had talked with other Republicans mulling campaigns. "I've got to go make another 10 or 12 calls," Tarr said.