Senator Barack Obama took a big step toward becoming the Democratic presidential nominee on Tuesday, amassing enough additional delegates to claim an all but insurmountable advantage in his race against Senator Hillary Rodham Clinton.While Clinton’s campaign continued to make a case that she could prevail, Obama seized on the results from Democratic contests in Kentucky and Oregon to move into a new phase of the campaign in which he will face different challenges. Those include bringing disaffected Clinton supporters into his camp; winning over elements of the Democratic coalition like working-class whites, Hispanics and Jews; and fending off attacks from Senator John McCain, the presumptive Republican nominee, especially on national security.Obama won easily in Oregon. But his obstacles were underlined by a lopsided defeat in Kentucky, where just half of the Democratic voters said in exit polls that they would back him in the general election this fall. Under the rules used by Democrats, the split decision was enough for Obama to secure a majority of the delegates up for grabs in primaries and caucuses. His campaign has portrayed success in winning those pledged delegates as the most important yardstick for judging the will of Democratic voters, and has encouraged superdelegates — elected officials and party leaders who have an automatic vote at the convention— to fall in line accordingly.“We have returned to Iowa with a majority of delegates elected by the American people, and you have put us within reach of the Democratic nomination for president of the United States of America,” Obama said in an address on Tuesday night, standing in front of a moonlit Capitol in Des Moines.Even as Obama moved closer to making history as the first black presidential nominee, he stopped short of declaring victory in the Democratic race, part of a carefully calibrated effort in the remaining weeks of the contest to avoid appearing disrespectful to Clinton and alienating her supporters. Instead, he offered lavish praise for his rival over 16 months.“Senator Clinton has shattered myths and broken barriers and changed the America in which my daughters and your daughters will come of age, and for that we are grateful to her,” Obama said. Clinton, declaring victory in Kentucky, made clear that she had no intention of stepping aside before the Democratic voting ends on June 3. “This is one of the closest races for a party’s nomination in modern history,” she said. “We are winning the popular vote, and I am more determined than ever to see that every vote is cast and every ballot is counted.”Going into Tuesday, Obama had 1,915 of the 2,026 pledged delegates and superdelegates needed to claim the nomination, according to a count and projection by The New York Times. His campaign estimated that if he simply held his own in the remaining contests, he would need only 25 more votes from superdelegates. There are 221 undeclared superdelegates left; Obama has been rolling out endorsements on a daily basis.But even as he moved closer to winning the intensely fought nominating contest with Clinton— a battle suffused with history and the tension inherent in a campaign defined in part by race and gender— Obama was preparing to deal with a series of challenges in the weeks ahead. He was planning a vigorous schedule of travel to general election states and a voter registration drive focusing on black voters to offset any losses among whites. Aides said he was considering delivering another speech to deal with damage in the primary because of attacks on his relationship with his former pastor, the Rev Jeremiah A Wright Jr, as well as on his patriotism. “We know we have our work cut out for us,” said Steve Hildebrand, a deputy campaign manager for Obama. “But we are up to the task.”At the same time, Obama’s aides said they were not concerned with exit polls showing that he had hemorrhaged white working-class voters to Clinton in Kentucky, mirroring similar findings in Indiana, Pennsylvania and Ohio. Many Clinton voters in Kentucky said they would stay home or vote for McCain in the fall. Two in 10 Democratic voters in Kentucky said race was a factor in their choice, and they overwhelmingly voted for Clinton. “You can’t look at it that way,” said David Plouffe, Obama’s campaign manager. “There’s enough evidence now in public polls that in a general election against McCain, in the states that will determine the presidency, her supporters are coming our way. I think this is an issue that in 30 or 60 days we will not be talking about.” Pledged DelegatesObama 50.7%1,649307 Superdelegates1,956 total70 needed to winClinton 46%1,497279 Superdelegates1,776 total250 needed to win