By Greg Giroux, CQ Staff

There’s a lot we don’t know about today’s long-awaited Democratic presidential primary in Pennsylvania — namely how many Pennsylvania Democrats will show up to vote and what vote percentages Hillary Rodham Clinton and Barack Obama will receive. Please visit our site this evening for the answers to those questions and analysis of the outcome.

What we do know, in advance of the 8 p.m. Eastern time poll closing, is that there are 4.2 million registered Democrats in Pennsylvania who are eligible to vote in today’s Democratic presidential primary, according to the office of the Pennsylvania secretary of state. And we know how these voters are distributed among Pennsylvania’s 67 counties. (Click here for a demographics chart.)

Nearly one in five Democratic registrants in Pennsylvania lives in Philadelphia, the most populous city in the state. More than 60 percent of the state’s Democrats are concentrated in 10 counties, most of which are in the Philadelphia or Pittsburgh region. All of them are briefly profiled below.

CQ Politics also has prepared a handy chart that includes political and demographic information about each of Pennsylvania’s 67 counties. Among the data included are the number of registered Democrats; the percentage of Democrats who are between the ages of 18-34 (a group that should be pro-Obama) and over age 65 (a group that should be pro-Clinton); median household income; educational attainment; and the racial composition of the population.

Pennsylvania counties with the most registered Democratic voters, with the largest one first:

• Philadelphia (799,663 registered Democrats; 19 percent of the total). A majority of Pennsylvania’s 1.2 million African-American residents live in Philadelphia, and this strongly Democratic-leaning voting bloc will vote heavily for Obama.

Most Philadelphia residents live in the 1st District, which is represented by Democrat Robert A. Brady , or the 2nd District, which is represented by Democrat Chaka Fattah . Both “minority-majority” districts are heavily Democratic and will vote for Obama, who seems assured of winning at least four of the seven district delegates in the 1st District and six of the nine district delegates in the 2nd District. (He needs to win 64.3 percent in the 1st to get a 5-2 delegate edge there, and 72.2 percent in the 2nd for a 7-2 delegate split.)

2. Allegheny (567,420 registered Democrats; 13.5 percent of the total). This county includes Pittsburgh and its suburbs and is the most populous area of western Pennsylvania.

3. Montgomery (247.881 registered Democrats; 5.9 percent of the total). Less than a decade ago, Montgomery — which abuts Philadelphia to the northwest — had 110,000 more registered Republicans than Democrats. Now the county has more registered Democrats than Republicans.

4. Bucks (185,407 registered Democrats; 4.4 percent of the total). This county north of Philadelphia comprises most of the population in the 8th District, which is represented by freshman Democratic Rep. Patrick J. Murphy , an Obama supporter.

5. Delaware (157,301 registered Democrats; 3.7 percent of the total). Most of Delaware County, which is southwest of Philadelphia and just northeast of Wilmington, Del., is in the state’s 7th District, which is represented by freshman Democratic Rep. Joe Sestak , a Clinton supporter. Fifteen percent of county residents are African-American.

6. Westmoreland (134,122 registered Democrats; 3.2 percent of the total). Westmoreland is an overwhelmingly white (97 percent) and mostly middle-class area that abuts Allegheny to the east and which includes the city of Greensburg.

7. Berks (114,304 registered Democrats; 2.7 percent of the total). This southeastern Pennsylvania county includes Reading.