(CNN) -- Sen. Bob Casey, a socially conservative Democrat and Pennsylvania's senior senator, announced his support for same-sex marriage Monday saying he had been influenced in part by "letters written to me by LGBT Pennsylvanians and their families."
"After much deliberation and after reviewing the legal, public policy, and civil rights questions presented, I support marriage equality for same-sex couples and believe that DOMA should be repealed," he said in a statement.
The letters, he wrote, "included deeply personal statements" and "had a substantial impact on my position on this issue."
"If two people of the same sex fall in love and want to marry, why would our government stand in their way? At a time when many Americans lament a lack of commitment in our society between married men and women, why would we want less commitment and fewer strong marriages?" he asked, underlining "less" and "fewer" to underscore his point.
Casey won re-election to a second term in November by eight percentage points and nearly 500,000 votes.
He is a social conservative and Catholic who campaigned on his support for civil unions but not marriage for gay and lesbian couples. His platform also included the conservative position of opposition to gun control.
Several Democratic senators endorsed same-sex marriage last week as the Supreme Court heard oral arguments in two cases related to the issue. Several Democrats from conservative or moderate states, including Sens. Kay Hagan of North Carolina, Mark Begich of Alaska, and Mark Warner of Virginia, have endorsed same-sex marriage.
Casey's announcement means only seven Democrats in the Senate are opposed to same-sex marriage, according to a Human Rights Campaign report last week which named Casey among eight senators opposed to same-sex marriage.
Only one Republican senator - Rob Portman of Ohio - has endorsed same-sex marriage. He said he was influenced on the issue by his son, who revealed to his father that he is gay.
-- CNN's Gregory Wallace and Paul Steinhauser contributed to this report