As Hillary Clinton and Barack Obama set out to work the crowds in delegate-rich Pennsylvania Tuesday, they were looking over their shoulders at the Mississippi primary, the last chance to pick up steam before the Keystone State puts its finger on the scale.

Polls have consistently favored Obama in Mississippi, which offers 33 pledged delegates and a large black electorate. Obama has won other southern states, including South Carolina and Georgia, with the help of black voters.

The Illinois senator was still trying regain his momentum after losing the Texas and Ohio primaries to Clinton one week earlier. He picked up a win in Wyoming Saturday and left nothing to chance in the Deep South, making a last stop in Greenville, Miss., before joining Clinton in Pennsylvania.

Obama visited Buck's Restaurant, signing autographs and joking with patrons.

"We just haven't seen as much opportunity come to this area as we'd like," he told a small gathering at Buck's restaurant. "And one of the challenges, I think, for the next president is making sure that we're serving all communities and not just some communities."

"I've been praying for you," a man called out.

"I believe in prayer," Obama replied.

Polls close at 8 p.m. ET.

Early turnout in the state was mixed, though, with the most people showing up in areas that offer both presidential and congressional primaries. Mississippians are voting on two rare open congressional seats.

In some areas, officials say morning rain may have slowed down participation.

Mississippi's secretary of state is predicting a light to moderate turnout. If the prediction of 125,000 to 150,000 participants holds true for the Democratic and Republican primaries, it would only be around 10 percent of registered voters.

Obama spent all of Monday in Mississippi, drawing enthusiastic crowds in Columbus and Jackson, the capital.

The Illinois senator began the night with 1,579 delegates, to 1,473 for Clinton. It takes 2,025 to win the nomination. With Mississippi, 602 pledged delegates are left to be allocated.

With both campaigns vowing to fight it out -- potentially to the last contests June 3, and beyond to the August convention -- the discord between the two campaigns has intensified since the March 4 primaries.

Obama on Monday rejected persistent suggestions by the Clinton campaign that he would make a great vice president to Clinton, saying, "I don't know how somebody who is in second place is offering the vice presidency to the person who is first place."

And the campaign on Tuesday called on Clinton to oust a prominent supporter for telling a California newspaper that Obama's positive media treatment has been race-based.

Former vice presidential candidate and U.S. Rep. Geraldine Ferraro, a member of Clinton's finance committee, made the remarks to The Daily Breeze, saying that Clinton suffers from a "sexist media," and that Obama would not be where he is if he were white.

The Rev. Jesse Jackson, who has endorsed Obama, praised Mississippi's primary as an example of how far the state has come from the racial and gender politics of the past, but worried about the ability of the two candidates' campaigns to pull together in the fall.

"My concern is they keep one eye on winning and one eye on reconciliation," Jackson told The Associated Press in an interview. "By the ending they must have one big tent to avoid a collision."

Former Gov. Ray Mabus campaigned with Obama Monday and Tuesday, and said in an interview he believed the campaign would do well.

Rep. Bennie Thompson, D-Miss., also joined Obama on Tuesday, and said the heavily black delta region would go overwhelmingly for the Illinois senator.

Blacks make up about 37 percent of the state's population, the highest concentration in the nation. Black voters accounted for more than half the vote in the state's 2004 Democratic primary.

Mississippi Democrats were energized by the Obama-Clinton rivalry, he said, whereas Republicans seemed less enthusiastic, in part because John McCain has locked up the GOP nomination.

With Clinton's camp saying she has little chance in Mississippi, the campaigning here focused largely on national issues.

Clinton held a rally in Harrisburg, Pa., Tuesday, where she touched on energy, trade and foreign policy, using the topics to slam Obama's judgment and veracity.

She accused him of voting to give tax subsidies to oil companies and "retard" the development of clean renewable energy in voting for a 2005 energy bill.

The Obama camp issued a response before Clinton even spoke those words accusing her of proving "she will say and do anything to win."

Spokesman Bill Burton said in the statement that Obama voted for the bill in order to raise taxes on oil companies and make investments in renewable energy.

Pennsylvania offers 158 delegates and is the largest remaining battleground for the Democratic candidates. Polls so far favor Clinton, and the New York senator is banking on a win there to make up ground in the delegate count.

Bill Clinton campaigned in Canonsburg, Pa., Tuesday, telling voters just as he did in Ohio and Texas that it's up to them to save her candidacy.

"I think she's got to win a big victory in Pennsylvania," he said. "I think if she does, she can be nominated, but it's up to you."

FOX News' Aaron Bruns, Bonney Kapp and Shushannah Walshe and The Associated Press contributed to this report.