Shifting the focus of the presidential race to the Iraq war, John McCain and Hillary Rodham Clinton bickered during Senate hearings about bringing troops home, while Barack Obama repeated his position that a dialogue must be held with Iran if the US seeks to improve the situation in Iraq.According to Obama, while he wants US troops out of Iraq, he would not initiate a precipitous withdrawal. And he said talking regularly to the Iranians is critical to getting to the point where it would be safe to end American involvement."I do not believe we are going to be able to stabilize the situation without them," Obama said.Commanding Gen. David Petraeus and Ambassador Ryan Crocker faced largely deferential questioning from Republican nominee McCain, who backs a continued US presence in Iraq, and Democratic rivals Clinton and Obama, both advocates of getting out of the oil-rich country after five years of war.Petraeus and Crocker, both highly respected for their conduct of the conflict since taking over last year, responded gingerly, knowing one the three senators was likely to be the next commander in chief.In the Foreign Relations Committee, McCain said promises to withdraw forces "would constitute a failure of political and moral leadership.""I fundamentally disagree," Clinton said later, when it was her turn to speak. "Rather, I think it could be fair to say that it might well be irresponsible to continue the policy that has not produced the results that have been promised time and time again."From his seat on the Foreign Relations panel, Obama prodded the two officials to redefine success as a means of closing down the conflict.The Illinois senator and Democratic front-runner said he worried that the goals - completely eliminating al-Qaida and Iranian influences - might be impossible to meet and troops could be there for 20 or 30 years in a fruitless effort."If, on the other hand, our criteria is a messy, sloppy status quo but there's not huge outbreaks of violence, there's still corruption, but the country is struggling along, but it's not a threat to its neighbors and it's not an al-Qaida base, that seems to me an achievable goal within a measurable timeframe," he said.Both Democrats contend the five-year-old war has made the United States less safe and is a crippling drain on the American economy. McCain said U.S. forces are succeeding.Petraeus also acknowledged in questioning from McCain and committee chairman Sen. Carl Levin, a Michigan Democrat, that the Iraqi military operation against militia forces in Basra late last month was poorly planned. He said Prime Minister Nouri al-Maliki, who ordered the assault, did not consult sufficiently with American forces."Suffice to say it was a disappointment," McCain asked.Petraeus responded: "It was. Although it is not over yet, senator."In opening remarks, Petraeus said he would recommend to U.S. President George W. Bush that the current drawdown of troops be put on hold for 45 days after July, when the so-called "surge" force is to have left the country. He refused to commit to a date for restarting the withdrawal.While McCain is virtually guaranteed the Republican spot in the race, Clinton and Obama continue battling in a close and historic battle for their party's nomination. She is seeking to become the first woman president; he aims to be the first black president.Obama has accumulated more delegates, won more primary and caucus contests and leads in the popular vote against Clinton. The latest AP tally of delegates showed Obama with 1,638 to Clinton's 1,501, including superdelegates _ party leaders and elected officials who are free to vote for whichever Democrat they want.On Wednesday, Obama will be returning to the campaign trail in Pennsylvania, which holds the next primary contest on April 22, offering the largest remaining prize of 158 delegates. Clinton hopes to replenish her campaign coffers with a fund-raising concert in New York featuring Elton John.A new Quinnipiac University poll of Pennsylvania voters showed Clinton's previously large lead over Obama had shrunk to 50-44. Clinton's lead has dropped from a 9-point advantage a week ago and 12 percentage points in mid-March.Seeking to assure a victory in the upcoming balloting, Clinton is targeting Pennsylvania media markets with five new television ads that deliver specific messages to different regional and ethnic audiences.Her campaign began airing the ads Tuesday, three in the expensive Philadelphia market where polls show rival Obama has been gaining support.The ads come as Obama has been outspending Clinton in Pennsylvania. As of Sunday, Obama had spent $3.6 million (â‚¬2.29 million) in the state to Clinton's $1.3 million (â‚¬830,000), according to data compiled by TNS Media Intelligence/Campaign Media Analysis Group.Obama updated his ad mix in the state as well. A new ad features some of the women in his life - his half sister, his grandmother and his wife - in what is an obvious outreach to women voters who form a core of Clinton's support.As the ads began airing, the Clinton campaign also issued a fundraising appeal to counter Obama's spending advantage in the state."They're trying to end the race for the White House with an unyielding media blitz," an e-mail to supporters says. "Don't let a sea of Obama ads overwhelm our powerful message in Pennsylvania. Contribute now."