A Senate Republican, Illinois' Mark Kirk, joined the fast-moving shift Tuesday, becoming the second GOP senator to do so. Ohio's Rob Portman was the first, on March 14.
"I've never seen a social issue change this quickly," said Rep. Rob Andrews of New Jersey, one of many Democrats across the country who voted for the act known as DOMA but who now support same-sex marriage.
After the flips by Casey and Carper, only seven Senate Democrats have not backed the idea. For Democrats, it is now the default position.
For most Republicans, though, the social debate and the way forward are far more murky. No Republicans representing the Philadelphia region have supported same-sex marriage, and they had little to say on the issue in response to Casey and others reversing course.
Several of the region's Republicans in Congress have previously voted for constitutional amendments to limit marriage to a man and a woman, a stand still favored by many social conservatives and a significant, but shrinking, swath of voters. None expressed any change in views the last two days, though some pointed to their support for civil unions and other gay-rights laws.
The GOP electoral base is still largely conservative on such social issues, even as polls repeatedly show the public moving in favor of same-sex marriage.
"I understand that the views of many Americans are changing on the issue of marriage," Rep. Charlie Dent (R., Pa.) of the Lehigh Valley wrote in an e-mailed statement.
That was the closest any local Republican came to delving into the changes around them. Counting Kirk, just four Republicans in Congress now support same-sex marriage, according to the Washington Post.
"For Republicans, it's a fine line to walk, because public opinion does seem to be shifting fairly quickly, but not really among their base," said Michele Swers, a Georgetown University political scientist.
The change has been dramatic. In 1996, 118 House Democrats joined 224 Republicans to support DOMA. Vice President Biden, then a Delaware senator, joined Pennsylvania conservative Rick Santorum in voting for the bill, along with 83 other senators.
In the House, Chaka Fattah (D., Phila.) was one of only 67 votes against the law, which President Bill Clinton signed - and has since repudiated. New Jersey Democratic Sens. Frank Lautenberg and Robert Menendez - then a House member - voted for DOMA. Both have since changed positions.
In 2004 the issue was still a political winner for the GOP, which used state-level marriage referenda to pump up turnout for President George W. Bush's reelection, Swers said.
But soon, Andrews said, cracks were widening in a dam holding back marriage equality.
As gay couples became more visible, Andrews said, lawmakers such as himself realized they knew such couples. "People began to look at the issue differently because they knew actual people instead of the stereotypes," he said.
Andrews said he regretted his own vote for DOMA and cast it "out of ignorance."
The dam burst, according to Andrews, after last year's election. President Obama backed same-sex marriage, and not only won reelection, but trounced Mitt Romney among young voters, who overwhelmingly back same-sex marriage.
"On came this torrent of people changing their minds," Andrews said.
Republicans can see the shifting demographics. But Swers said their political base consisted largely of older, white voters, those most likely to oppose same-sex marriage.
"This issue is a microcosm of the present crisis of the Republican Party," Andrews said. "They're being torn in two directions: by their primary electorate on the one hand, and the general electorate on the other."
Swers said Republicans had tried to focus on other issues. "You don't see them loudly speaking against it," she said. "They're just kind of quiet about it."
Indeed, asked for their latest positions, area Republicans had little to say Monday and Tuesday. Of eight GOP senators and representatives from the region, three reiterated previous stands opposing same-sex marriage; aides for four did not reply to requests for comment or said the officials were unavailable.
Sen. Pat Toomey (R., Pa.), who, as a U.S. House member, supported a 2004 constitutional amendment that would have limited marriage to a man and a woman, included that principle in his 2010 Senate campaign. He has not changed his views, a spokeswoman said.
Dent supported a similar amendment when it came up in 2006. So did Reps. Frank LoBiondo and Chris Smith, both New Jersey Republicans. The two also backed DOMA.
Reps. Jim Gerlach (R., Pa.) and Mike Fitzpatrick (R., Pa.) were two of only 27 Republicans to vote against the 2006 amendment, but each has said marriage should be between a man and a woman. So have Reps. Pat Meehan (R., Pa.) and Jon Runyan (R., N.J.), each elected in 2010.
"As Rep. Meehan has long said, he believes marriage is between a man and a woman," a spokesman wrote Tuesday, adding that civil unions should be decided by states.
Gerlach "has been consistent in his support of the existing federal law that defines marriage as the union of one man and one woman," a spokesman wrote, stressing that Gerlach supports civil unions.
Aides to LoBiondo and Runyan said the lawmakers could not be reached. Spokesmen for Fitzpatrick and Smith did not return e-mails seeking comment.
Aides to Runyan and LoBiondo also pointed to those lawmakers' support for civil unions.
Some Republicans, including LoBiondo and Dent, highlighted antidiscrimination bills they have backed.
Contact Jonathan Tamari at firstname.lastname@example.org follow on Twitter @JonathanTamari. Read his blog, "Capitol Inq," at www.philly.com/CapitolInq.