The Illinois senator leads Clinton by 23 percentage points among white men and by 17 points among liberals - groups that were evenly divided between the two in early February. He has a similar advantage among people earning $50,000 to $100,000 annually - whom she led earlier by 13 points.
Clinton maintains robust leads among some groups that have been cornerstones of her candidacy, including those age 65 and up, white women and people earning under $50,000 annually.
Overall, Obama has 46 percent to Clinton's 43 percent, a virtual tie. Clinton had a slight 5 point lead nationally in early February.
Obama's advances among voting groups are even more stark when viewed longer term.
In mid-January - when primaries and caucuses were just beginning - Clinton had a 7-point lead among all men, a group she now trails by 25 points. The two were about even among college graduates, whom Obama now leads by 20 points.
A majority of Democrats in the poll also said elected officials and party leaders who may play a pivotal role in choosing the candidate should back the one who leads in the primaries and caucuses. As of now, that would be Obama.
These so-called "superdelegates" may be crucial because neither Obama nor Clinton may accumulate the delegates needed to ensure the nomination at the August convention in Denver. Allowed to support whomever they like, these officials have been under enormous pressure from both campaigns.
In the AP-Ipsos poll, 57 percent said these officials should support the contender ahead in the voting while 38 percent said they should back whoever they think gives Democrats the best chance of winning the White House. Of those saying the officials should follow the voters, two-thirds said they should heed nationwide results while a third preferred basing it on the officials' own districts.
Obama's supporters were likelier than Clinton's to favor following the voting results so far.
Among the nearly 800 superdelegates, Clinton leads in endorsements 241 to 182, according to the latest tally by the Associated Press. But Obama has won more delegates in primaries and caucuses, giving him the overall delegate lead, 1,370 to 1,274.5.
In matchups looking ahead to November's general election, Clinton leads Arizona Sen. John McCain by 48 percent to 43 percent. Obama's lead over the virtually certain GOP nominee is twice that size, 51 percent to 41 percent.
Obama does better than Clinton does against McCain among better educated, moderate and middle-class voters, winning among those groups while Clinton would split them with the Republican. Obama trails among the oldest voters while Clinton and McCain divide them evenly.
The two Democrats do about equally against McCain among several groups that lean toward Clinton, leading him by similar margins among women, the least educated and lowest-income voters. McCain is ahead against both Democrats among whites, while they lead him easily among minorities.
In the GOP race, McCain leads Mike Huckabee, his only remaining major rival, by 53 percent to 27 percent, about twice the margin he had over the former Arkansas governor just two weeks ago. McCain has all but formally clinched the nomination. Texas Rep. Ron Paul had 8 percent.
McCain has large leads over Huckabee among people calling themselves loyal Republicans, conservatives and Southerners, a measure of his growing acceptance among groups that initially were skeptical of him.
The poll showed the two are about even among white evangelical Christian voters who attend church at least weekly, a group Huckabee - a Baptist minister - has dominated in GOP voting. Their equal standing in Monday's survey may indicate that such voters who oppose McCain have been more motivated to actually show up and vote, but McCain's standing among them also rose in the poll from two weeks ago.
The AP-Ipsos poll was conducted from Feb. 22-24 and involved telephone interviews with 1,011 adults nationally. The margin of sampling error was plus or minus 3.1 percentage points.Included were interviews with 473 Democratic voters and people leaning Democratic, with a margin of sampling error of plus or minus 4.6 points; and 381 Republicans or GOP-leaning voters, with a margin of sampling error of plus or minus 5 points.
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AP News Survey Specialist Dennis Junius and AP writer Stephen
Ohlemacher contributed to this report.
