Sen. Hagan is a top target of Republicans in 2014. (United States Senate/Wikimedia Commons)

Hagan, who announced her support via Facebook post, became the fourth Senate member of her party this week to make such a revelation, leaving only nine of the 53 Senate Democrats yet to endorse marriage equality, according to the Washington Post. Senate Republicans, who tend to be more socially conservative, have been slower to move on this issue, although Ohio Sen. Rob Portman revealed his support for same-sex marriage last Friday in a Columbus Dispatch op-ed. Portman wrote that his evolution was heavily influenced by his son Will, who told his parents he was gay two years ago.

Out of this week’s four new supporters, three, Hagan, Sen. Jon Tester of Montana and Sen. Claire McCaskill of Missouri are from states Republican presidential candidate Mitt Romney carried. The fourth, Sen. Mark Warner represents Virginia, which voted for President Obama two times but before that last went for a Democrat in 1964.

This dramatic shift by relatively electorally vulnerable Democrats highlights the move in public opinion on this issue. According to a Gallup poll taken last December, a record 53 percent of Americans support legalizing same-sex marriage.

After yesterday’s oral argument on California’s Proposition 8, which banned same-sex marriage in the state, the Supreme Court Wednesday heard a challenge to the Defense of Marriage Act, which prevents the federal government from recognizing same-sex marriages recognized by individual states.

Hagan currently occupies the Senate seat that was held for 30 years by the late extreme social conservative Jesse Helms, who once called gays “weak, morally sick wretches.” She faces a likely tough re-election campaign in 2014.

Read more of Neon Tommy’s coverage of Proposition 8 here.

Reach Executive Producer Matt Pressberg here.