Voters in both groups are energized: Blacks by the early successes of Illinois Sen. Barack Obama, Latinos by the intense, sometimes xenophobic debate over immigration. But it's far from clear how those influences will play off each other.
Nevada's caucuses on Jan. 19 will give an early showcase of Hispanic voting. However, observers say the true impact of Latino influence might not be felt until the general election, notably in Western states like Colorado, New Mexico and Nevada -- places where George Bush's margin of victory in 2004 was razor-thin.
When South Carolina Democrats hold their primary on Jan. 26 -- the state GOP contest is Jan. 19 -- the choices of substantial numbers of black voters will be tallied for the first time in this election.
Obama's stunning victory over Hillary Rodham Clinton in the Iowa caucuses and strong second in New Hampshire's primary showed he could win white votes. But some say the South Carolina contest offers a new test of his viability: Can he energize black voters in places where their numbers could help him win in November?
Race has played a key role in American politics for as long as there have been Democrats and Republicans.
Fred Garrett, a black South Carolinian, recalls how his parent's voted Republican, the party of Lincoln, before the Great Depression. But when Franklin Roosevelt offered a New Deal, they took it and shifted their loyalties to the Democrats. Most blacks who overcame social and legal barriers to voting cast their ballots the same way.
By some measurements, Garrett -- an evangelical, churchgoing, anti-abortion social conservative with a successful mortuary business in the GOP stronghold of Greenville -- would make a natural Republican. But he says the GOP left him and his black brethren "by the wayside" long ago, and he doesn't see any evidence that that will change anytime soon.
"I never have voted Republican nationally," says Garrett, 83, whose first ballot was for FDR as a Navy enlisted man toward the end of World War II. "I started to vote for Eisenhower one time, but I didn't."
In 1956, Republican Dwight D. Eisenhower was able to garner 39 percent of the black vote, notes Donald Bositis, a senior research associate for the Joint Center for Political and Economic Studies, a black think tank in Washington, D.C.
But with the rise of newly converted Republicans like Sen. Strom Thurmond and their efforts to thwart civil rights legislation, the GOP could manage only 6 percent of the black vote in 1964.
"And that's when the change was over," says Bositis.
The historical association between the Democrats and the working class, coupled with the election of John F. Kennedy as the first Roman Catholic president, accounts for the Latino affiliation with that party -- Florida Cubans being the great exception. Democratic candidate Bill Richardson, who cited JFK as one of his inspirations, showcased his Hispanic roots before he pulled out of the race Thursday.
"The vast majority of Hispanics were, are and remain working class," says Gary M. Segura, an associate professor of American politics at the University of Washington. "And so, not surprisingly, that means that they have economic interests which are historically more coincident with the Democratic Party than with the Republican Party."
According to the Pew Research Center, Hispanics are twice as likely to identify themselves as Democrat than Republican. For blacks, it's 101/2 times.
"There is in the United States a racial tone to the political parties," says Bernard N. Grofman, director of the Center for the Study of Democracy at the University of California, Irvine. It's something "that nobody wants to talk about very much, because in some ways it's really very, very embarrassing."
Both minority groups lag behind whites in voter registration. The latest census figures indicate that while 71 percent of voting-eligible whites are registered, the rate drops to 61 percent among blacks and 54 percent for Latinos.
The conventional wisdom has been that as the nation's population moves toward a minority majority, its political complexion will become more Democratic. Or, as Grofman puts it, the "browning of America will result in the bluing."
But in studying the South, Grofman -- author of the voting-rights history "Quiet Revolution in the South" -- found a correlation between the percentage of a state's black voting population and increases in white support for Republican candidates.
Grofman notes there have been small but measurable Latino shifts toward the GOP as Hispanic homeownership rates, conversions to evangelical Protestantism and generational distance from immigration increase. And since many Latinos identify racially as white, he says we may see a "mimicking" of the electoral "white flight" from the Democratic Party he identified in the South.
A Hispanic-black divide is already showing in the nomination battle.
A California poll by the Field Research Corp. found Clinton's lead over Obama had dropped from 25 percentage points in October to just 14 points late last month. However, the same survey gave Clinton a 20-point lead among Latinos, who comprise 14 percent of voters there.
Segura's polling in Nevada showed heavy support for Clinton among likely Latino voters there, too.
In 2004, George Bush reached out to Hispanic voters and got a GOP-record 44 percent of the Latino vote by some exit poll estimates. But he won Nevada by around 20,000 votes, and the Democrats have been registering Latinos there by the thousands.
The first two Fridays of each month, the Democratic party sets up voter-registration tables outside the federal court chamber in Las Vegas where new citizens are sworn in.
"We average about a hundred every Friday," says Andres Ramirez, Latino outreach coordinator for the state party. "From time to time, we'll get a thousand a week."
Latino registration rates in the state have risen from just 4 percent in 1996 to more than 10 percent. Given the "very anti-immigrant" stances taken by the state GOP, which adopted an English-only platform that would deny citizenship to U.S.-born children of illegals, Ramirez is confident that most of those political newcomers will be voting Democratic.
Albert Ambiz, a naturalized citizen born in Guadalajara, Mexico, is registered Republican. But this week, as he watched Clinton canvass his heavily Hispanic neighborhood on Las Vegas' East Side, he said though he was still leaning Republican, he wasn't sure which party he would caucus with on the 19th.
The 38-year-old electrical foreman, stepping off the sidewalk to let Clinton and a pursuing pack of reporters pass by, said on immigration, his party and its messengers were guilty of "Hate, to the core."
"They see us as just taking from the system," he said. "But the reality is, even the illegals, they do put in more then they take out."
If Obama wins the Democratic nomination, Segura and others wonder what effect "black-brown competition" will have on the Latino vote this fall. Segura agrees with Grofman that it's dangerous to assume the two groups will complement each other at the ballot box.
"It's not clear that there would be a lot of enthusiasm for an African-American from a Latino electorate," he says.
Vanderbilt University Law School professor Carol M. Swain, author of "Black Faces, Black Interests," is one African-American who won't be voting for Obama -- nor, likely, for any Democrat. She says none of the party's candidates has articulated a position "that really takes into consideration the harm that's being done to working-class Americans" by competition from illegal immigrants.
She doesn't feel "that shared race is a strong enough position to support a candidate."
But experts say many black voters may take the opposite tack when they cast ballots in South Carolina, where blacks make up about half of the Democratic electorate.
Donald Aiesi, a political science professor at Furman University in Greenville, S.C., thinks turnout in the party primary there will be 4-to-1 black.
And he predicts that "the race pull" will be strong -- even though, he adds, "I don't think anybody's going to talk to a pollster or anybody else and say, `Well, with me it's ultimately the idea that my son or daughter could be elected."'
Not all black voters agree. John H. Corbitt, Garrett's pastor at the Springfield Baptist Church, doesn't see it that way. Iowa notwithstanding, he's leaning toward Clinton -- partly because former President Bill Clinton was so good to blacks, but mostly because he thinks the New York senator can win in November. "Many of us are tired of being on the losing side."
Garrett, the mortician, says it's time to try something really new, and that's looking more and more like Obama.
"He's saying the things I want to hear," Garrett says. "I know he won't carry through all of them, but he'll carry through some of them. And it will be beneficial to our people."