"It's time for the commonwealth to be on the right side of history and the right side of the law," state Attorney General Mark Herring said at a news conference.
Herring, a former state senator, stressed marriage equality as part of his campaign last year. His narrow election and Terry McAuliffe's victory as governor marked a major change in Virginia politics with Democrats replacing conservatives in top executive positions.
Former Virginia Attorney General, Republican Ken Cuccinelli, who lost the gubernatorial election, was a strong opponent of legalizing same-sex marriage.
Multiple lawsuits in Virginia charge that the Old Dominion's same-sex marriage ban violates the Constitution's equal protection and due process clauses.
The Democratic position rankles the GOP in Virginia, a so-called "purple state" where the general assembly is dominated by Republicans and policies on social values trend conservative.
"By running for the office, Mark Herring asked for the challenge of defending Virginia's constitution and all it contains," Republican Party of Virginia Chairman Pat Mullins said in a statement.
"If Mark Herring doesn't want to defend this case, he should resign, and let the General Assembly appoint someone who will. Mark Herring owes the people of Virginia no less," he added.
Partisan politics at the state level have helped shape the national debate on key issues, including same-sex marriage, abortion, gun rights, and voter identification.
Larry Sabato, director of the Center for Politics at the University of Virginia, said official positions in many areas can shift abruptly when party control of government changes.
"In this polarized political era, a great gulf separates the two parties on all the hot-button social issues," he said.
Reversal of the same-sex marriage ban in a crucial battleground state nationally follows court decisions striking down similar prohibitions in Utah and Oklahoma, states where Republicans dominate the legislature and the governor's office.
President Barack Obama won Virginia in 2008 and 2012 and lost Utah and Oklahoma in those same elections.
Fifty-seven percent of Virginians voted to approve the same-sex marriage ban in 2006. But recent polling indicates that a slight majority now support the right of gays and lesbians to marry.
"This lawlessness is an insult to the voters of Virginia who approved the marriage amendment by a large majority," said Tony Perkins, president of the Family Research Council. "The 'left' is becoming a law unto itself."
A state's hue makes a big difference on where it stands on issues like same-sex marriage, abortion, gun rights and voter identification laws.
There are currently more Republican governors than Democrats and more Republican dominated legislatures.
For example, the 17 states and the District of Columbia that allow same sex-marriage are all considered Democratic-leaning "blue states." With a few exceptions, most states with a same-sex marriage ban tend to be Republican-leaning "red states."
Also, more than half of the nation's state legislatures introduced measures last year that sought to sidestep any potential federal ban on firearms, assault weapons and high-capacity ammunition magazines, according to data collected by the National Conference of State Legislatures.
Most of those were so-called "red states" with Republicans in charge of at least one chamber of the legislature and often governorships.
Moreover, several Republican-dominated states with a history of voter discrimination pressed forward with controversial voter identification laws after the Supreme Court struck down a key part of the Voting Rights Act.
And the strongest resistance to participating in insurance exchanges, a key component of Obama's signature health care reform law, came from states with Republican governors and GOP-led legislatures.
"This underlines how much elections matter," Sabato said.
By Paul Steinhauser and Halimah Abdullah