Montgomery County Commissioner Jim Matthews, a Republican, and newly elected Commissioner Joe Hoeffel, a Democrat, are expected to announce a deal this morning to share some of the power at the county courthouse in a move that will undercut incoming Republican Commissioner Bruce L. Castor Jr.

Under the plan, Matthews would become chairman of the Board of Commissioners and Hoeffel would become vice chairman, according to a Democratic strategist. Though Republicans would retain most top positions in county government, Democrats would be placed in second-rank positions in some offices.

“Full inclusion” is how one long-term GOP official described it last night.

The unusual alliance stems from the troubled relationship between Matthews and Castor, who gave up his job as district attorney to run for commissioner.

“They can’t stand each other,” said one official, speaking on condition of anonymity. “Neither wants the other to be chairman.”

Castor disagreed. “I like Jim Matthews, and if he wants to be chairman, there are a number of circumstances in which I would agree to support him,” he said last night.

An alliance between a Republican and Democrat would give the Democrats real power in the county courthouse after failing to win control at the ballot box in November.

Hoeffel, a former congressman, confirmed late yesterday that he and Matthews would hold a news conference this morning in Norristown “to announce our plans for the future of the county government.”

“We’re going to focus on the government side of things, not the politics, and move forward,” said Hoeffel, who declined to comment further.