Kach said the change in heart came after considerable deliberation. He plans to put out a statement at 10:15 a.m.
The move puts O'Malley closer to the 71 votes he needs for passage in the 141-member chamber, but it is still unclear that supporters have the numbers they need. Lawmakers will begin debating the marriage bill this morning. A final vote could come as soon as Friday.
Kach's decision is stunning since he voted against the same-sex marriage bill in committee on Tuesday night and is a co-sponsor of a different measure that would define marriage as between a man and a woman.
He is the second House Republican to support the measure. Another lawmaker, Robert Costa of Anne Arundel County, voted for the bill in committee on Monday night.
In his statement, Kach said that "no one event or conversation prompted me to come to this decision." He added that he was "significantly moved" by the testimony of gay and lesbian families raising children.
The full text of his statements is as follows:
Over the span of my career in the Maryland General Assembly, I have worked hard to take thoughtful, considered positions on laws that affect the rights and liberties of our friends and neighbors." “My constituents sent me to Annapolis to represent them and use my best judgment. They did not send me to sit in judgment of the lives of others."
"As a proud member of the party of Lincoln, I believe that we as legislators should be more concerned with relieving the tax burden of families than telling them how to behave in their own homes."
"Like so many others, my thoughts on the issue of civil marriage have evolved over the course of recent months as a result of much reflection and listening to good people on both sides of this issue. Instrumental to my decision are the enhanced protections for churches, clergy, and faith leaders in my community and in communities around the state."
"While no one event or conversation prompted me to come to this decision, I was significantly moved by the testimony of families -- who are raising children in a loving environment and deserve every right to enjoy the same protections and responsibilities that our laws provide for others."
"Tomorrow I will cast my green vote with great confidence that this bill protects religious freedom and that the issue will ultimately be decided by the voters of our state."