Over two days, lawyers from both sides of the emotionally-charged debate over marriage equality will put their cases before the nine justices who make up the highest court in the land.
Outside the Supreme Court, color-coded protesters -- red for the pro-equality side and patriotic red, white and blue for opponents of gay marriage -- will set out their positions in the court of public opinion.
Opinion polls have repeatedly indicated that a majority of Americans now accept the principle of same-sex marriage, which is legal in nine states plus the District of Columbia but banned or limited in 41 others.
But opposition -- spearheaded by social conservatives and backed by the Roman Catholic and evangelical churches -- remains strong, with many contending that marriage can only be a union between a man and a woman.
President Barack Obama restated his support for same-sex marriage on Monday though a Twitter account managed in his name by Organizing for Action, an advocacy group founded after his re-election.
"Every American should be able to marry the person they love," read the tweet, which carried the hashtag "#LoveIsLove."
Tuesday will see the Supreme Court hear arguments over Proposition 8, a California referendum measure that struck down that state's same-sex marriage initiative in 2008.
The two couples who are plaintiffs in that case staged a photo-opportunity on Monday by inspecting the original handwritten text of the US Constitution on the eve of their historic day in court.
Paul Katami and Jeff Zarrillo joined Kris Perry and Sandy Tier on the steps of the National Archives, where they posed for news photographers but declined to speak to reporters.
They then went in "to view the US Constitution and reflect on the importance of their case for gay and lesbian couples across the nation," said the American Foundation for Equal Rights, which supports their case.
On Wednesday, the Court will consider the Defense of Marriage Act (DOMA), a federal law which defines marriage as between a man and woman and thus denies married gays and lesbians the same rights and privileges.
The star plaintiff in that case is Edie Windsor, 83, who was obliged to pay more than $600,000 in federal estate taxes upon the 2009 death of her longtime partner Clara Spyer, who she had married in Canada in 2007.
Under DOMA, the surviving member of a heterosexual married couple is exempt from such taxes.
The Human Rights Campaign, a leading gay rights group, is urging supporters to dress in red for a major rally on the Supreme Court steps Tuesday "to show your support for marriage equality."
Opponents of same-sex marriage meanwhile plan their own "march for marriage" from the National Mall to the imposing Supreme Court building -- with women invited to dress in red, children in white and men in blue.
"We believe it is imperative that political leaders, the media, and the culture see that we care about protecting marriage enough to stand up and march for it," said its organizers, the National Organization for Marriage.
Overnight vigils for marriage equality were meanwhile being held throughout the United States, and the Los Angeles Times reported that a lesbian cousin of Chief Justice John Roberts would be in court on Tuesday.
Roberts, nominated to the court by former president George W. Bush, played a key role in the Supreme Court's decision to uphold Obama's controversial health care reform package.
Another critical member of the bench is Anthony Kennedy. A conservative, he has upheld LGBT rights in the Supreme Court, but also warned against democracies depending on their judiciaries to sort out their political rows.