But earlier Tuesday Clinton expressed dismay at the peculiar voting rules in Texas, where Democratic delegates are allocated through a combination of the results from primary votes and caucuses. Texas apportions delegates in a complex system that may yield Obama more delegates from expected wins in Texas' big cities, while giving Clinton less delegates for expected wins in Latino areas along the border.
"When the dust clears, we have to ask some tough questions," Clinton told reporters in reference to the Texas voting rules, arguing that the limited window to appear at a caucus makes it difficult for particularly working class voters to participate.
Obama went into Tuesday's contest with a 110-delegate lead, according to the ABC News delegate scorecard.
Clinton has a very serious math problem. Almost regardless of what happens Tuesday and in the few remaining states left to vote, she will be behind Obama in delegates when the last primary vote is cast in Puerto Rico in June. That makes the role of superdelegates all the more important.
The Obama campaign issued a statement Tuesday suggesting Clinton must make a significant dent in Obama's pledged delegate lead -- an unlikely event given state polling and the Democratic Party's proportional system for according delegates.
"The Clinton campaign said this race was all about delegates and that they would be tied or ahead by morning," said Obama spokesperson Bill Burton in his statement. "But despite the 20-point lead in Ohio and Texas that Senator Clinton had just two weeks ago, we will still be well ahead in delegates tonight and they will have failed at achieving their plainly stated goals."
Obama began election day in Houston at a livestock and rodeo exhibit, posing for pictures and climbing on a tractor.
En route to San Antonio, Obama and his wife, Michelle, strolled to the back of the plane, telling reporters he thought the race is "tight" and described Clinton as a "tenacious" candidate.
Obama said he was surprised the media "bit" on her "complaining about the refs" -- referring to Clinton's complaint during a debate in Ohio that the media was giving preferntial treatment to Obama.
With her candidacy at stake, Clinton has turned up the heat on Obama in recent weeks, questioning his national security and foreign policy credentials and his position on trade.
The Clinton campaign launched a television ad on Friday asking voters to consider which candidate they would want to pick up the phone in a time of crisis.
"It's 3 a.m., your children are safely asleep. Who do you want answering the phone?" it said.
The Clinton campaign also held a conference call to urge reporters to probe more deeply into Obama's relationship with Tony Rezko, a Chicago real estate developer who is currently on trial in connection with extortion allegations.
On Tuesday as voters headed to the polls, Clinton communications director said Obama's campaign hasn't been transparent with Ohio voters about a conversation Obama's senior economic adviser Austan Goolsbee had with a Canadian official in Chicago.
A Canadian government memo reports Goolsbee told the Canadians Obama's anti-NAFTA rhetoric should be taken more as political positioning than an articulation of policy objectives. The Obama campaign and Goolsbee dispute the Canadian memo.