Neall, a fiscal moderate who served as Anne Arundel County executive and was often mentioned as a possible gubernatorial candidate, has told friends he has grown uncomfortable in a Republican Party that has moved increasingly to the right.
Neall declined to discuss his plans, saying he would issue a statement tomorrow.
But he made clear that he has grown weary of what he termed "intraparty criticism" from more conservative members of the Republican Party.
"There have been lots of times in my political career when some have said I'm either not conservative enough or Republican enough," Neall said.
"I think it's worsened in recent years."
Neall said he has visited or called several political friends in recent days to inform them of his intentions.
"I've got people I need to contact and talk about it ahead of time," he said. "I've thought about this for a long time. It's time to make a decision."
Three sources confirmed that Neall has said he will switch to the Democratic Party.
Neall, 51, is considered one of the General Assembly's authorities on state budget matters, a gifted speaker and a master of parliamentary tactics.
"The Maryland Democratic Party would welcome his views."
Sen. Martin G. Madden of Howard County, the Senate Republican leader, said he met with Neall on Tuesday and tried to persuade him to keep his party affiliation.
"We're doing all we can to convince him that his political home should be here," Madden said. "Until he's made up his mind and publicly announced it, we will continue to try."
Neall would become the second Republican elected official to bolt the party in less than a month, following Jack A. Gullo Jr., the 31-year-old mayor of New Windsor.
But unlike Gullo, little known outside Carroll County, Neall was a party standout with a long history of service in Annapolis.
Paul Ellington, executive director of the Maryland Republican Party, said he was not aware of Neall's plans but said his criticisms of the party were misplaced.
"The Republican Party's been good to Bob Neall," Ellington said. "It was the same Republican Party that put him in that seat.
"The ones he's turning his back on are the ones who elected him."
Neall's switch to the majority party would likely have little impact on his influence within the General Assembly.
Senate President Miller appointed Neall to the three-member conference committee to represent the Senate in final negotiations over the state budget the past two years -- an influential role unusual for a member of the minority party.
Miller moved to deny rumors sweeping through political circles that he promised Neall a coveted committee position within the Senate if he became a Democrat.
"He hasn't been pushed or pulled or tugged or promised anything," Miller said.
Political observers said that in becoming a Democrat, Neall might be angling for a better shot at statewide office, such as Maryland comptroller.
As a young man, Neall was a Democrat but switched to the GOP in the early 1970s when he went to work for the 10-member Senate Republican caucus -- a group dominated by moderates, such as Newton Steers of Montgomery County and Jervis S. Finney of Baltimore County.
Beginning in 1975, Neall served 12 years in the House of Delegates, becoming a key voice on fiscal matters and leading the Republican caucus from 1982 to 1986.
In 1986, Neall narrowly lost to Tom McMillen in a bid for the U.S. House of Representatives. He returned to elective office in 1990 as Anne Arundel County executive.
He served one four-year term before turning to lobbying and other business endeavors.
In 1996, he was tapped to fill the seat in the Maryland Senate left vacant at the death of his mentor, John A. Cade.
Neall quickly assumed Cade's role as the pre-eminent Republican voice on the budget and many other matters. Last fall, he won election to a full four-year term, easily turning back a challenge from a more conservative Republican in the process.
But even as he won a full term, two of his moderate GOP colleagues in the Senate, F. Vernon Boozer of Baltimore County and John W. Derr of Frederick County, were defeated by more conservative Republicans.
Sen. Barbara A. Hoffman, chairwoman of the Senate budget committee, of which Neall is a member, said he has wrestled with his decision in recent weeks.
"I think he feels like he's letting down the few remaining moderate Republicans in Anne Arundel County," said Hoffman, a Baltimore Democrat. "But he feels like he's not welcome."