SAN ANTONIO — Hillary Rodham Clinton, once seen as a lock for the Democratic nomination, battled yesterday in possibly the last weekend of her presidential campaign, struggling to reverse a tide of money and momentum that has turned dramatically toward Barack Obama.

The New York senator stormed across Texas, questioning Obama's readiness to lead, particularly on national security issues. "You are in effect hiring the next president," Clinton told supporters at a rally at a San Antonio high school. "What you've got to decide is: Who do you want to hire?"

The Illinois senator touched down in Rhode Island — his first campaign visit to the tiny state — as well as Ohio.

Obama targeted Clinton with some of his harshest criticism of the campaign, knocking her for taking money from federal lobbyists, voting for "George Bush's war in Iraq" and voting in favor of a bankruptcy bill that made it "harder for families to climb out of debt."

The three states and Vermont will vote Tuesday in contests that could effectively settle the Democratic fight — or extend the race well into springtime or beyond.

In a campaign that has frequently defied expectations, a consensus emerged as the candidates caromed across the country: Clinton must win Texas and Ohio to have any serious hope of sustaining her bid to become the nation's first female president. A split decision would not suffice, analysts said, and winning narrowly may not help.

"We're reaching a point where — not all voters, but lots of voters — are starting to feel it's time for the party to coalesce around a candidate," said Geoffrey D. Garin, a veteran Democratic pollster who is unaligned in the contest. "The Clinton campaign has to have a compelling and persuasive reason to go on. ... She's got to come out of Tuesday with people believing that she has a realistic path to the nomination."

The political math seems to work against the former front-runner. Obama has opened a small but growing lead of delegates to the Democratic National Convention. Unless Clinton starts winning big — and polling in Texas and Ohio suggests that will be difficult — she could have a tough time overtaking Obama.

"We have to maintain our delegate lead and make sure that we don't get blown out in those two states," Obama told reporters earlier this week as he campaigned across Texas. "If we come out of the four contests on Tuesday with a gap in the delegate count of 100 or 150, which we have right now, then I continue to believe that we will go to the convention with the most earned delegates and believe that we should be the nominee."

After reeling off 11 consecutive victories, Obama has two things going for him as he vies to become the nation's first black president: the proportional awarding of delegates — which means he can keep adding to his number even if he loses the popular vote to Clinton — and the campaign calendar.

Obama has been careful to avoid suggestions he was taking the nomination for granted and turning to the November race against presumed GOP nominee John McCain.

On Friday, at an American Legion post in Houston, a man in the audience said he hoped that Obama, in the "ensuing debates" with McCain, would make clear his opposition to the concept of pre-emptive war.

"First of all, just one point I've got to make is I still got this thing called a Democratic primary," Obama said. "I don't want to jump the gun. ... I've got this very tough competitor named Hillary Rodham Clinton."