Lyndon Johnson is famously quoted as observing, “There goes the South,— as he signed the Civil Rights Act. Interestingly, Vice President Joseph Biden is said to have had a very different take on what happened on July 2, 1964.

He viewed it as the liberation of the national Democratic Party from the programmatic and structural domination of Southern conservatives (and sometimes racists) and the condemnation of the Republican Party to a historical asterisk. The political metamorphosis of Sen. Arlen Specter (Pa.) is but the latest evidence that the vice president is far more on target than the conventional wisdom that has dominated American politics for a half-century.

The Congressional wing of the Democratic Party in the first half of the 20th century, up to the signing of the Civil Rights Act, was dominated by conservative Southern Brahmins. A coalition of conservative Republicans and Southern Democrats had inordinate control of the House and Senate through the Truman, Eisenhower and Kennedy presidencies. These conservative Democrats, with their Republican allies, dominated the committees and shaped the national agenda.

What changed in 1964? The national electoral map inverted. The solid South stayed solid, but, beginning with the regional hemorrhage to Barry Goldwater in 1964, became the electoral, and more significantly the ideological, base of the GOP.

As Southern voters became increasingly and reliably Republican, and as the number of Southern House Members, Senators and governors increased, the power of Southerners in the Republican Party leadership surged. The libertarian, internationalist Republican Party began to wither on the vine. The metamorphosis in both parties took a half-century to solidify, but today the new electoral map outside the South is decidedly and increasingly Democratic, and even in the South there is a demographic foreboding for the GOP.

The pre-1964 great Republican Party of New England and the Northeast is gone, self-immolated. The classic Republican moderate Senators have been eaten by their own. The giants — Clifford Case, Jacob Javits and Ed Brooks — were all defeated by internal right-wing challenges reflecting the ideology of the new Southern Republican base. There are no Republican Members of the House from New England.

The elegant and powerful Republican Party of New York that produced President Theodore Roosevelt and Vice President Nelson Rockefeller controls no statewide office, runs neither house of the state Legislature, can barely muster one-third of the popular vote for president, and has but three of the state’s 29 House seats. The reliably Republican electoral college bases in California, Illinois and Ohio are now blue, as is all of the Pacific West, all of the industrial Midwest (now even including Indiana), and all of the Mid-Atlantic states.

The same pattern holds for Congressional races. The recent special election win by Democrat Scott Murphy (N.Y.) in what, by party registration, should be solidly Republican 20th district is further evidence of the dissipation of the Republican Party as a competitive force in the Northeast. Increasingly the only meaningful elections there are within Democratic primaries.

Analyst (and Roll Call contributing writer) Stuart Rothenberg traces the problem back to the inflexible doctrinaire ideology of the grass-roots Republican base.

“They’re just so ideological, they continue to believe that they need to be more conservative to win elections, where most strategist believe in the Northeast, you need moderate Republicans to appeal to more moderate voters,— he recently told CNN. “So it’s really a question of, do you understand that this is a very big country with different kinds of people in different parts of the country.—

The base of the Republican Party is not only increasingly uni-regional; it is demographically contracting. It is literally dying as we speak. Old white men voting reliably Republican are being replaced by young people of all colors voting reliably Democratic. Even the Republican Cuban fortress in Florida has become a generational trap, with young Cubanos shifting to the Democratic Party and thus delivering the state to Barack Obama in November.

The Hispanic vote is flipping the reliably Republican states of Colorado, New Mexico, Nevada and Arizona to the Democrats. The surge of minority voters, young voters and college and post-college graduates tilted even two other once solidly Republican states: North Carolina and Virginia. The increasingly diverse Georgia and Texas are probably next on this inevitable demographic shift. The marginalization of the Republican Party is both geographic and demographic.

Why? A central reason is that the Civil Rights Act took the monkey off the Democrats’ back. The party no longer had to be prisoner to the ideology (and often, unfortunately, the racism) of the South. It was free to reach out and surge with minorities, young people, academics and labor.

Maryland Democratic Sen. Barbara Mikulski’s observation that “demography is destiny— is as true of politics as of anthropology. Between the ideological liberation of the 1964 epiphany and the demographic explosion of progressive constituencies, America appears headed into a prolonged and sustained Democratic era.

Lyndon Johnson was right about the Civil Rights Act and the South, but only in the short term and he seemingly missed the greater significance of that regionalization to the future of American politics. Contrary to the conventional wisdom of a half-century, July 2, 1964, may well be remembered not as the day the Democratic Party died in the South, but rather the day the GOP began to die as a national political party.

Mark Siegel is a partner in Locke Lord Strategies and a former executive director of the Democratic National Committee.