As Democrats head to the polls in the crucial Pennsylvania primary Tuesday, a much clearer view of the U.S. electoral puzzle is starting to emerge from the fog of Campaign 2008.

Four groups of voters -- working-class males, young people, rural and small-town Americans and Hispanics -- stand out as the key pieces of that puzzle. All four groups are in flux, and they will provide the leading indicators of where the race is heading.

The role of these key voting blocs will be much in evidence in Pennsylvania, a state that in many ways is a microcosm of the U.S. A surge of newly registered voters in the state likely includes a major wave of young residents; that figures to benefit Sen. Barack Obama, who has a sizable lead among younger voters in the polls.

Sen. Hillary Clinton, in turn, is counting on a strong performance in the rural and small-town environs of south central Pennsylvania, where polls show her doing well. Meanwhile, working-class white males are a swing constituency in the blue-collar areas around Pittsburgh, and they have been enthusiastically courted by both candidates.

The Pennsylvania outcome will go a long way -- conceivably all the way -- toward determining which of the two Democrats will prevail and move on to the November general election. Sen. Clinton needs to win, and probably by a comfortable margin, to maintain hope of prevailing against Sen. Obama.

But regardless of which Democrat prevails, these four groups will be in the cross-hairs of both parties for the rest of the year.

Here's why:

Working-class white males. Forget soccer moms; it's football dads who are more interesting this year.

The general exodus of white working-class voters of both sexes from the Democratic Party was a well-documented cause of the party's decline during the past quarter century. A combination of cultural issues and economic dissatisfaction drove both Joe and Jill Sixpack toward Republicans. President Bush also benefited from a feeling among working-class women that he could keep their families safe from terrorism. He won the white working-class vote by 23 percentage points in 2004, according to research by Ruy Teixeira, a voting scholar at the Brookings Institution. But two years ago, amid dissatisfaction with the war in Iraq and what Mr. Teixeira calls "a loss of faith that Republicans knew how to run the economy," Democrats began to claw back, particularly among women. In the 2006 congressional elections, Republicans won among white working-class women by just five percentage points, down from 15 percentage points two years earlier, Mr. Teixeira says.

But Democrats didn't regain the same kind of ground among white working-class men, who still voted Republican by a 14-point margin in 2006.

So what now?

The advantage Democrats have is that economic anxiety among this group of men is high and growing. The problem Democrats have is that Sens. Obama and Clinton have to reach across racial or gender barriers to win them over.

As a veteran and blunt-speaking war hero, Sen. John McCain, the Republican candidate, has a kind of natural affinity with these football dads. A look inside the Wall Street Journal/NBC News poll frames the battle. While working-class white males are almost evenly divided between Democrat and Republican in party identification, a survey last month showed Sen. McCain beating Sen. Obama among them by 17 percentage points, and Sen. Clinton by 25 points.

In many ways, these voters are the classic Reagan Democrats, or traditional Democrats first lured away from the party by Ronald Reagan, and a new Quinnipiac University poll of Pennsylvania shows Sen. Clinton leading among them by 49%, compared with 44% for Sen. Obama.

Young voters. The excitement surrounding the Democratic primary campaign, and particularly Sen. Obama, has attracted waves of young voters onto voter-registration rolls. An analysis by Public Opinion Strategies, a Republican polling firm, showed that in the first 18 states that held primaries this year, the number of voters ages 18 to 29 voting in the Democrat contests nearly tripled from four years ago.

The classic question about young voters, though, is a simple one: Can they be counted on to show up on Election Day?

The rise in both registrations and primary-election turnout by young voters certainly suggests the possibility of a big showing this year. In a sign of that potential, turnout by voters under age 30 four years ago rose faster than among any other voting group, according to data compiled by the nonpartisan group Rock the Vote.

Yet even with that uptick, young voters turned out in lower proportions than any other age group. Turnout among those under 30 was 49%, compared with 73% of those age 60 to 74, the Rock the Vote data show.

The Quinnipiac survey of Pennsylvania shows Sen. Obama leading among Democratic voters under the age of 45 by a 57%-to-41% margin. But the real turnout test will come in November.

Rural voters. Rural and small-town voters are the best indicators of whether a candidate is connecting with the values of Middle America. "They are America," says Peter Hart, a Democratic pollster who helps conduct the Journal/NBC News poll. "Too often Democrats end up with candidates who can speak only to metro America. If you can speak to [rural and small-town America], then you relate to the rest of America."

Republicans have long enjoyed an advantage with these voters. Moreover, this is the slice of the electorate where Sen. Obama's recent remark that small-town voters "cling" to guns and religion because they are frustrated economically could hurt.

For Sen. McCain, his showing among these voters will indicate whether social and cultural appeals continue to resonate with Middle America. The Journal/NBC News poll shows that among rural and small-town voters nationwide, 45% have a positive view of Sen. McCain, compared with 38% for Sen. Obama and 32% for Sen. Clinton.

These voters matter in important swing states, including Ohio and Missouri. They also are important in Pennsylvania. The Quinnipiac survey shows Sen. Clinton leading among rural Democratic voters there by 58% to 37%; among suburban Democrats, by contrast, her lead is 49% to 47%.

Hispanics. The importance of the growing Hispanic population has been much discussed in recent years. What's less understood is just how heavily Sen. McCain's campaign is banking on a strong showing among Hispanics.

Research by the Pew Hispanic Center has shown that, in the wake of tough anti-immigrant rhetoric from many Republicans, Hispanic feelings toward the Republican Party overall have grown frosty since Mr. Bush won 40% of the Hispanic vote in 2004. But Sen. McCain, who has never bought into his party's toughest anti-immigrant rhetoric, stands out as the Republican presidential aspirant with the strongest standing among Hispanics.

So if Sen. McCain can match, or even approach, the level of support Mr. Bush achieved among Hispanics, he will stand a much better chance in the "Big Four" Western swing states of Arizona, Colorado, New Mexico and Nevada. Look for an awful lot of outreach to Hispanics from Sen. McCain.

Capital Journal -