HARRISBURG, Pa. — Democrats have assumed the majority in the Pennsylvania House and promptly elected a Republican as speaker the first time in at least a century that a majority party doesn’t have one of its own members as presiding officer.

Republicans had held the majority for 12 years, but Democrats picked up eight seats in November, just enough to give them a 102–101 lead.

That was followed by two months of ballot recounting and political intrigue that culminated in a behind-the-scenes deal in which nearly all House Democrats and a half-dozen Republicans banded together to oust the Republican speaker, John Perzel, and put the gavel in the hands of Rep.

Dennis O’Brien, a Republican, by a 105–97 vote.

Mr. O’Brien was generally viewed as a less divisive figure than Mr. Perzel, particularly among Democrats, and his promises of reforms drew in votes from Republicans looking for an alternative.

The Democratic leader, state Rep. Bill DeWeese, had agreed to step aside when it became apparent that a defection among his party’s ranks made it difficult for him to win a head-to-head matchup with Mr. Perzel.

Mr. O’Brien, 54, took over Tuesday.

“You have my pledge. I will move reform issues forward and I will try to be as fair as I possibly can,” Mr. O’Brien told the chamber.

The public’s desire for substantial reforms in the legislative process and state government was underscored by election fallout from the legislature’s 2005 deadof-night vote later rescinded to increase its members’ own salaries. Twenty-two House incumbents were defeated last year, and 28 others opted to retire.

The two highest-ranking state senators also were defeated.

One of the six Republicans who supported Mr. O’Brien, state Rep. David Steil, said the prospect for reform was why he broke ranks.

“I took a chance, because I wasn’t convinced of the total support for some of the rules changes we wanted to make,” Mr. Steil said. “I don’t know that we can get to 102 votes without the support of leadership.”