Republican Charlie Baker's decision to run for governor days after state Treasurer Tim Cahill made a move toward an independent bid sets up a potentially titanic three-way race with incumbent Deval Patrick, local political activists said Tuesday.

"This will be one of the biggest battles gubernatorially we have ever seen," said state Sen. James Timilty, D-Walpole.

Baker, a former state government official, is stepping down as head of Harvard Pilgrim Health Care to seek the Republican nomination for governor in next year's state election. Businessman Christy Mihos is also running as a Republican.

Republicans have eyed Baker as their dream candidate for years, saying he has the combination of business and government experience a governor needs.

"This is exciting. He would be a fantastic governor," state Rep. Jay Barrows, R-Mansfield, said of Baker. But a third-party bid by Cahill could draw support away from Baker because voters disenchanted with Patrick could split between the two, although Barrows said he believes Cahill could eventually decide against running for governor.

Timilty said that under normal circumstances an independent candidate helps the incumbent and hurts the challenger, but next year might be a unique election season.

He said he is torn on whom to support because he has worked well with Patrick, but has been friends with Cahill since 1996.

Adding to his dilemma, he said, is state Sen. Scott Brown, R-Wrentham, could possibly be Baker's running mate, and he and Brown are friends.

Brown said he is unsure what he will do about running for higher office.

"I'll just do my job and whatever happens, happens," he said.

Patrick has been taking a beating in the polls as the recession and resulting budget problems have brought down his standing among voters.

He has also had some high-profile fights with his party's leadership in the Legislature and suffered some political gaffes.

But, his core of supporters among activists in the Attleboro area remain with him, and he has kept in touch with meetings and e-mails.

State Rep. Steve D'Amico, D-Seekonk, admitted Patrick "shoots himself in the foot" on occasion, but said he is sticking with the governor because "I agree with him more than I disagree with him." He said Cahill strikes him as the type of politician who would say anything to get elected.

Democratic activist Darren Major of Attleboro, one of the first in the area to support Patrick in 2006, said he admires Patrick for staying on the job and fighting budget problems when he could have abandoned the state like previous governors have done.

He said Patrick could have gotten a job in the administration of President Barack Obama, but stuck with his commitment to Massachusetts.

Jim Humphrey, chairman of the Attleboro Democratic City Committee, said he supports Patrick because he has accomplished so much in three years.

Patrick has helped implement the state's universal health care program that is a national model, got a $1 billion bill promoting life science industries passed and forced the Legislature into approving ethics, transportation and pension reforms.

JIM HAND covers politics for The Sun Chronicle. He can be reached at 508-236-0399 or at jhand@thesunchronicle.com.