WASHINGTON — For years, the United States has been drifting further apart, less a single country than an uncomfortable marriage of vastly disparate cultural and political entities, a Red America and a Blue America with starkly different realities on masks and vaccines, gun rights and voting rights, Donald J. Trump and the legitimacy of the 2020 election.

Now the chasm may open even wider. If the Supreme Court overturns Roe v. Wade, as it appears poised to do, all 50 states will suddenly be free to set their own rules, leading to one America where access to an abortion is guaranteed and another where it is outlawed — and, in some cases, helping someone cross state lines to obtain one could become a crime.

Already in the days since the leak of a draft ruling reversing Roe, governors and state legislators have rushed to define the values of their separate Americas. While Gov. Gavin Newsom of California vowed to amend his state’s constitution to protect abortion rights, Gov. Kevin Stitt of Oklahoma signed new legislation prohibiting abortion after six weeks. Calls for state legislatures to address the issue in special sessions proliferated on both sides of the divide.

The map showing both the states ready to ban abortions if the Supreme Court lets them and the states building protections for the procedure into their own laws looks strikingly familiar in this season of schism. It would fit neatly atop maps showing state policies on the pandemic or state crackdowns on critical race theory or, for that matter, the Electoral College map of recent presidential elections. The populous Northeast, mid-Atlantic seaboard and West Coast form one like-minded bloc, while the South and most of the Mountain West form another, with the Midwest split between them.

“It’s two different worlds — hostile, suspicious of each other and assuming bad intent,” said Mike Murphy, a veteran Republican strategist and the co-director of the Dornsife Center for the Political Future at the University of Southern California, which has studied political polarization. “It’s become totally tribal. There are no opponents anymore. Everyone is an enemy.”

To the extent that President Biden was elected promising to bring the country together, a hope that both liberals and conservatives considered naïve, he appears to have put that pledge to the side, at least for now. In the wake of the leak of the draft abortion ruling, he has assailed the Trump faction of America in a way he has mostly avoided until now.

“This MAGA crowd is really the most extreme political organization that’s existed in American history, in recent American history,” Mr. Biden said this week. Jen Psaki, his press secretary, reinforced that on Friday, citing Republican efforts to outlaw abortion. “His view is that is another example of the ultra-MAGA agenda as well,” she said.

Mr. Biden still boasts of the bipartisan support he secured for last year’s public works spending package, as he did on Friday in Ohio, where he praised Senator Rob Portman, a retiring Republican, and recalled the days when senators from both parties could debate civilly. “Things have changed,” he said. “We got to bring it back.” But aides noted that, with control of Congress on the line in this fall’s midterm elections, he had a responsibility to wage a vigorous campaign explaining to the public the consequences of changing parties on Capitol Hill.

The emerging White House strategy is to refocus attention away from high inflation by drawing a strong contrast with Mr. Trump’s party, warning that turning out Democrats will put Congress in the hands of the party of far-right figures like Representative Marjorie Taylor Greene of Georgia, a fiery, onetime QAnon follower known for making racist and antisemitic remarks and for casting doubt on the terrorist attacks of Sept. 11, 2001.

Conservatives quickly seized on Mr. Biden’s language this week to accuse him of betraying his own promises. “President Unity Declares War on Half of America,” Breitbart News, the far-right website, pronounced in a headline on a home page that regularly wages war on the other half.

Five of the last six presidents talked about healing America’s divide without much success. George H.W. Bush called for a “kinder and gentler nation,” Bill Clinton promised to be the “repairer of the breach,” George W. Bush termed himself “a uniter not a divider” and Barack Obama declared there was not a Blue America and Red America but “the States of America.”

David Axelrod, who was Mr. Obama’s chief strategist, said if he could talk with himself in 2004, when the future president famously made that speech at the Democratic National Convention, he would say, “It’s harder than it looked.”

Society has only been torn further apart in the last 18 years, both by cultural shifts as well as by the political and financial incentives for exploiting divisions, he said. “The reversal of Roe would accelerate these divisions and widen that chasm in ways that seemed unthinkable in 2004,” Mr. Axelrod said.

Mr. Trump, of course, was the presidential outlier, the only occupant of the Oval Office in modern times who hardly even paid lip service to the notion of bringing the country together. Instead, he relentlessly promoted divisions, seeking to punish Democratic-voting states and describing Blue America as a dystopian hellhole for which he had no responsibility.

The United States has rarely been as united as its name; polarization has been part of the country’s DNA from the beginning, erupting most explosively in the middle of the 19th century over slavery and again in the middle of the 20th century over desegregation. But even so, the last couple of decades have seen a fragmentation that, by some measures, has been among the most pronounced in American history.

One study found that Democrats and Republicans in Congress are further apart ideologically than at any point in the last half-century. The public’s view of its presidents has grown more divided along partisan lines than at anytime in the history of polling. House districts have grown so rock-solid liberal or conservative that only a few dozen will be truly competitive in this fall’s election.

“Really, in every area of politics, you see evidence of partisan polarization,” said Carroll Doherty, director of political research at Pew Research Center.

Increasingly, Americans are separating into their own safe spaces — geographically, culturally, ideologically, factually and metaphorically. Not only do they stick to news channels or social media accounts that reinforce their viewpoints, they choose to live among and socialize with those who share their opinions.

In 1960, 4 percent of Democrats and Republicans said they would be unhappy if their children married someone from the other party. Today, according to the Public Religion Research Institute, that number has grown to 35 percent among Republicans and to 45 percent among Democrats. Over the course of just four years, the Institute for Family Studies found, marriages in America between Republicans and Democrats fell by half. As it was, in 2016 only 9 percent of marriages involved couples from opposite parties; by 2020, that figure had slid to just 4 percent.

Lilliana Mason, a political scientist at the SNF Agora Institute at Johns Hopkins University, said her research shows that Americans likewise do not even want to live next door to someone from the other party. “Our realities become different. The people we surround ourselves with have completely different narratives about what’s happening in America,” she said.

Ms. Mason, who on Friday published her latest book, “Radical American Partisanship: Mapping Violent Hostility, Its Causes, and the Consequences for Democracy,” written with Nathan P. Kalmoe, said the fragmentation of abortion laws in a post-Roe America would only exacerbate those trends as people sought to live in states where they agreed with the new laws.

“The fact that we’ve physically moved away from each other allows us to hate each other more,” she said. “It’s easy to dehumanize someone you’ve never met. It encourages the us-versus-them sort of thinking that creates this dire stakes for elections — if they win the election, everything is over.”

Americans’ views of abortion are actually more nuanced than the black-and-white politics surrounding the issue would suggest. Today’s Republican candidates and officeholders are less willing to support exceptions for rape or even to protect the health of the mother, while today’s Democratic politicians are less likely to support limits on taxpayer funding for abortions.

But a new study released on Friday by the Pew Research Center showed that while a strong majority of Americans opposed repealing Roe, their attitudes fractured depending on the question. Only 19 percent supported abortion being legal in all cases, while 42 percent wanted it to be legal in most cases but would accept some instances when it would be illegal. Only 8 percent wanted it to be illegal in all cases, while 29 percent wanted it illegal in most cases but would accept some circumstances when it would be legal.

“This is an issue where there are some absolutists — people who say abortion should be legal in all places or abortion should be illegal in all places — but most Americans have less absolutist views on this,” said Jocelyn Kiley, the Pew center’s associate director of research.

Nuance, however, is not the order of the day. In Blue America or Red America, the loudest voices tend to dominate the conversation. The reversal of Roe, should it happen, presumably will fuel that trend. “It’s a really polarizing feeling,” said Ms. Mason. “It’s a massive disagreement that Americans are having right now.”