When the US supreme court struck down the federal definition of marriage as between a man and a woman in a historic ruling last year, it left open the question of the constitutionality of similar state laws and, with it, the door to a host of legal challenges.

Over the past few weeks, the momentum that kicked off with America's highest court's recognition of same-sex marriage in June has accelerated rapidly through the lower courts. The end of last year and beginning of 2014 have seen a series of favourable civil rights decisions by federal judges over challenges brought by gay and lesbian couples living in some of the 33 states with same-sex marriage bans, ruling them unconstitutional.

States that voluntarily embraced marriage equality tended to be clustered in the north and north-east, such as New York, Washington DC and Massachusetts. But since last year's ruling, court challenges are gathering pace in traditionally conservative states with voter-approved bans on same-sex marriage. Last week, Virginia became the first southern state ever to have its voter-approved prohibition on same-sex marriage overturned. The recent election of a Democratic governor and attorney general accelerated the moves there.

Change is happening so rapidly that gay rights advocates say they expect a wave of legal victories they hope will continue all the way to the supreme court.

The marriage project national director at Lambda Legal, Camilla Taylor, said: "We've reached a watershed where public officials feel ashamed of discriminating against people and are sensitive to the fact that they could go down in history as being on one side or the other of a struggle for civil rights."

According to Taylor, the "game changer" was the decision in United States v Windsor, the case of an octagenarian lesbian widow who sued to claim federal estate tax relief, which resulted in the supreme court striking down part of the federal Defense of Marriage Act.

Taylor said: "The Windsor decision added tremendous momentum. It's this feeling, this expectation of success for the plaintiff couples in these lawsuits. This is something that the courts take into account as well. That isn't to say we won't see a few outliers. I think the majority of court decisions will vindicate the gay and lesbian civil rights movement."

There are more than 20 federal cases charging ahead in which same-sex couples are challenging either their state's ban on same-sex marriage or a failure to recognise a legal marriage from another state. Still more are challenging related rights, such as adoption for same-sex couples.

Taylor said the rulings and the failure of public officials to defend what they describe as discriminatory laws show a consensus that will hold water when a case gets to the supreme court. She said: "The supreme court does not like to weigh in too early or take a position too far in advance of public opinion. It likes to sweep in the recalcitrant states on national consensus. And more and more we are seeing a national consensus. These rights are being vindicated in the north-east, in the heartlands, in the south-west and soon, they will be vindicated in the south."

In her ruling to strike down Virginia's same sex voter-approved marriage ban last week, Judge Arenda Wright Allen agreed with legal analysis by federal judges in Utah and Oklahoma, both conservative states – Oklahoma is known as the "buckle" of the Bible belt. All three rulings found that the bans violate the due process and equal protection clauses of the US constitution's 14th amendment.

Also last week, a judge in Kentucky ruled that the state must recognise legal same sex marriages performed in other states and in Nevada, the state attorney general, a Democrat, and its governor, a Republican, said they would no longer defend their state's same-sex marriage ban.

Both Utah and Oklahoma states are appealing, seeking to reinstate the ban. They have been joined by five religious groups, including Mormons, Catholics and evangelicals.

All of the courts that have been faced with the issue since June have ruled that state bans on same-sex marriage or failure to recognise a legal same-sex marriage from elsewhere is unconstitutional.

A professor of law at the University of California, Douglas NeJaime, said these "are all states that politically weren't going to move to same-sex marriage any time soon. The courts are forcing their hands. We seem to have all of the lower courts going in the one direction in saying that Windsor applies to these state laws so that the states have to recognise same-sex marriage."

Not including Utah and Oklahoma, 27 states still have constitutional prohibitions on same-sex marriage. Four more – Indiana, Pennsylvania, West Virginia and Wyoming – do not permit it through state laws.

Ken Connelly of Alliance Defending Freedom, a legal group that has defended traditional marriage across the US, took the opposite view. He said he expected the rulings in favour of same-sex marriage to be overturned on appeal.

He said: "We would say that each one of the courts misreads Windsor. No one can claim that there's anything unusual in Oklahoma's marriage laws or in Utah's marriage laws. Windsor affirmed the sovereignty of the states like New York or other states to define marriage as they see fit."