"Melting Pot" conveys the image of many many different cultures all blended into one English-speaking American. "Mosaic" is now the "politically correct" usage to convey the image of people who are native speakers of another language and who also learn English and are Americans.
David Dinkins used "gorgeous mosaic" in his successful 1989 campaign for mayor.
(Oxford English Dictionary)
melting pot, n.
fig. A situation in which things are constantly changing and the outcome is uncertain; a place where different peoples, styles, theories, etc., are mixed together.
[1782 'J. H. ST. J. DE CRAVECUR' Lett. from Amer. Farmer iii. 39 Here [i.e. in America] individuals of all nations are melted into a new race of men, whose labours and posterity will one day cause great changes in the world.]
1887 Pall Mall Gaz. 10 Feb. 3/1 Mr. Morley..figured as a Conservative defending the Constitution from the melting-pot.
1909 I. ZANGWILL Melting-pot 37 America is God's Crucible, the great Melting-Pot where all the races of Europe are melting and reforming!
2 September 1889, New York (NY) Times, "Canada's Home Trouble":
In view of the virtual extinction of the French in the Mississippi Valley the French Canadians had a misgiving that if they too were cast into the American melting pot they would yield to that mysterious force which blends all foreign elements into one homogeneous mass.
28 June 1891, Los Angeles (CA) Times, pg. 4:
"As it was in the plain of Shinar, so it will be in the United States if the melting-pot of the common school is not able to fuse all the heterogeneous multitude of men and women who know not the English tongue, into an entirely new nation of English-speaking people."
5 August 1908, New York (NY) Times, pg. 5:
Other plays in his budget are "The Melting Pot," a new drama by Israel Zangwill, ...
18 November 1956, New York (NY) Times, pg. 22:
"MIXED" OR "MELTED"?
TO THE EDITOR:
Professor Allan Nevin's incisive article on immigration, "Epic of Liberty Island," (Oct. 28) is a potent answer to those persons who call for uniformity in America today.
I believe the supposedly innocuous epithet, "melting pot," which is often applied to this nation, has many of the connotations of homogeneity which Professor Nevins so expertly criticized. The idea of being able to "pour" the contrasting people who make up this country into a mold is what frightens me.
Another eminent historian, Professor James Blaine Hedges, of Brown University, saw this problem in the same light as the author of this article. Professor Hedges used to call America the great "mixing bowl." Judge by the history of this nation, I think it is quite true that the United States has not been a "melting pot," but is indeed a "mixing bowl."
Ithaca, N. Y.
11 April 1965, Washington (DC) Post, pg. E4:
The Canadian Mosaic
(...) The groups, sometimes called New Canadians by the English and Neo-Canadiens by the French, are strongly protesting the limitation of Canadian cultures to two. The study should be of multiculturalism, not biculturalism, they say, noting that they, too, are contributing to what some of them call the "Canadian mosaic" (as opposed to the familiar phrase across the border "the melting pot").
19 June 1988, New York (NY) Times, pg. BR22:
Mosaic vs. Melting Pot
(...) "Canada holds the myth of the mosaic," she said, "rather than the melting pot."
26 October 1989, New York (NY) Times, pg. B2:
In a phrase he repeats so often on the stump that reporters roll their eyes and mouth the words, Mr. Dinkins says New York is not a melting pot but a "gorgeous mosaic" of groups and interests. After long years of waiting, he seems eager to preside over that mosaic.
8 November 1989, New York (NY) Times, pg. A30:
New York City, whether melting pot or mosaic, has now conferred its collective respect on another branch of the American family. By electing David Dinkins as Mayor, New York has released a wave of pride in its citizens of African-American heritage and a sense of unity among the many white voters who rallied to his cause.
New York (NY) Times
The Mosaic Thing
Published: January 3, 1990
First there was the melting pot, but that seemed overly romantic. Then came patchwork quilt, a down-home image that Jesse Jackson has used over the years. Now the metaphor of the moment is gorgeous mosaic, Mayor David Dinkins's favorite description of New York City. ''I see New York as a gorgeous mosaic of race and religious faith, of national origin and sexual orientation,'' he said in his Inaugural Address on Monday, turning his oft-used campaign slogan into - a trademark, the equivalent of George Bush's ''kinder, gentler nation.''
Where did ''the mosaic thing,'' as it has become known at City Hall, come from?
When he ran for mayor of New York in 1977, Mario Cuomo called the city a ''magnificent mosaic,'' and recalls saying the same about the entire country three years before that. ''I never liked 'melting pot,' '' he says. ''Our strength is not in melting together, but in keeping our cultures.'' Mr. Dinkins, who first described the city as a mosaic when he ran for Manhattan borough president in 1985, says he never liked the idea of denying cultural differences either.