“Houston, I think we achieved liftoff here,” Mr. Obama told a crowd of 20,000 people in that city as he hailed the voters of Wisconsin. “The change we seek is still months and miles away, and we need the good people of Texas to help us get there.”

Image Senator Barack Obama at a rally in Houston on Tuesday night. Credit... Rick Bowmer/Associated Press

With 90 percent of the electoral precincts in Wisconsin reporting, Mr. Obama had 58 percent of the vote to Mrs. Clinton’s 41 percent. On the Republican side, Mr. McCain had 55 percent to Mr. Huckabee’s 37 percent. And early returns in Washington State showed him with 48 percent of the vote to Mr. Huckabee’s 21 percent. In Hawaii, Mr. Obama had 75 percent of the vote, with 71 of precincts reporting, while Mrs. Clinton had 24 percent.

In Wisconsin, the survey of voters leaving the polls found that Democrats believed Mr. Obama would be more likely than Mrs. Clinton, by 63 percent to 37 percent, to defeat the Republican nominee in the fall.

Her latest losses narrowed even further Mrs. Clinton’s options and leaves her little, if any, room for error. Her road to victory is now a cliff walk.

By the calculation of her own aides, she now almost certainly will need to win the next two big contests, Texas and Ohio on March 4, as well as Pennsylvania on April 22 in order to maintain a viable claim to the nomination and stop so-called superdelegates from breaking for Mr. Obama. But there has been evidence this month that Mr. Obama is building momentum with each victory, and recent polls have suggested that Mrs. Clinton’s once-large lead in Ohio and Texas is shrinking.

What is more, it may not be enough at this point for Mrs. Clinton to simply win Ohio and Texas. She needs delegates to catch up with Mr. Obama; under the rules by which the Democratic Party allocates delegates, she will need to win double-digit victories to pick up enough delegates to close the gap.

Finally, Mrs. Clinton continues to struggle to find a way to try to raise questions about Mr. Obama and stop what has been a rush of voters to his side. Her Tuesday night speech about Mr. Obama’s experience level was one of her toughest yet; still, she has been making similar arguments for months now, and they have not caught fire thus far.