Amid signs Democrats are increasingly ready for the bitter race to end, Pennsylvania voters are making their choice Tuesday for the party's next presidential nominee.

Polls opened this morning across the state and remain open until 8 p.m. ET, after last-minute campaigning by Barack Obama and Hillary Rodham Clinton. The Republicans also hold a primary today, but John McCain has clinched the party nomination.

"What (Obama) has to demonstrate is to win a big state, a big state that Democrats need to win in order to achieve the presidency," Clinton told CBS' The Early Show, repeating what has become a common theme in her campaign. "The road to Pennsylvania Avenue for a Democrat goes right through Pennsylvania."

Obama, noting Clinton's polling lead, sought to lower expectations.

"I think we've trimmed that back, but our view has always been that we're the underdogs here," he said. "I think she has to be heavily favored to win."

Monday, on the eve of their first contest in six weeks, the two Democratic rivals angled to manage expectations. Obama insisted he wasn't expecting a Keystone State win, while a new USA TODAY/Gallup Poll showed him with a lead nationwide over Clinton, 50%-40%, among Democrats and independents who lean Democratic. The sample of 552 adults has a margin of error of +/—5 percentage points.

In Pennsylvania, a rash of last-minute state polls showed Clinton anywhere from 10 points ahead to three points behind — with most giving her single-digit leads.

"I'm not predicting a win," Obama told Pittsburgh radio station KDKA. "I'm predicting it's going to be close and that we are going to do a lot better than people expect."

The poll also found that Democrats and Democratic leaners are evenly split 48%-48% on whether the long, heated nomination contest is hurting the party and party leaders should unite behind a candidate. A Rasmussen Reports poll last month found 62% of Democrats wanted the race to go on.

Despite concerns about the Democratic race dragging on, both candidates lead presumptive Republican nominee John McCain, according to the latest USA TODAY/Gallup Poll. The matchups are within the margin of error, +/—3 percentage points. The telephone poll of 1,016 adults was taken Friday through Sunday.

Clinton began the final day of campaigning in Scranton, her father's hometown, and traveled to Pittsburgh and Harrisburg before ending it in Philadelphia.

She repeated a favorite theme on readiness, portraying Obama as untested. "If you can't stand the heat, get out of the kitchen," she told a Pittsburgh audience. "I am very comfortable in that kitchen."

Obama held a discussion on economics with 50 voters in Blue Bell, part of the suburban Philadelphia area he is looking to win big. He then flew west for a town hall in McKeesport and a rally at the University of Pittsburgh. Both candidates were joined by their spouses.

The last-minute push included taped messages from both on World Wrestling Entertainment's Monday Night RAW on USA Network. Obama scheduled an appearance on Comedy Central's Daily Show with Jon Stewart, and Clinton gave an interview to Larry King on CNN's Larry King Live!

Clinton, trying for a decisive victory here to keep her nomination hopes alive, released a TV ad that featured images of Osama bin Laden, World War II and economic threats from such forces as soaring gas prices. The ad asks voters who they think can "stand the heat" of being president. Obama responded with a TV spot asking viewers who will be a uniter and "not use fear and calculation to divide us." Neither ad mentions the rival by name.

The dueling ads reflected the sharpness of the final days on the trail. Clinton suggested a vote for Obama amounted to "a leap of faith or guesswork." Obama said a vote for Clinton was a vote to continue counterproductive game-playing in Washington.

The negative tone, combined with an ABC debate last week that was heavy on character and scandal questions aimed mostly at Obama, have led many party strategists to conclude it's time to end what satirist Stephen Colbert calls "Democralypse Now — the delightful dismemberment of the Democratic hopescape."

Bill Carrick, an unaligned party strategist in Los Angeles, called last week's debate "Exhibit A in why this thing needs to end. Everybody feels the same. Enough already."

Democratic Party Chairman Howard Dean has said he wants superdelegates — the party leaders and elected officials not bound by primary or caucus results — to start choosing sides now. Dean said he doesn't anticipate that the nomination will be settled until after the final nominating contests June 3.

Clinton joked with a crowd in Harrisburg that the nomination fight "has been the world's longest job interview. … But we're getting down to a final decision."