WASHINGTON — With Senator Clinton needing decisive victories in today’s Texas and Ohio primaries to significantly narrow Senator Obama’s delegate lead, the question surrounding her campaign has become: What will it take for the former first lady to drop out of the presidential race? The two Democratic White House hopefuls exchanged attacks yesterday over trade and foreign policy as their campaigns sought to set expectations for a critical series of contests.

Polls show the race virtually tied in Texas and Mrs. Clinton maintaining a narrow lead in Ohio. She had topped Mr. Obama by more than 20 points in both states before the Illinois senator ran off with 11 consecutive victories over the past month. Voters also head to the polls today in Rhode Island and Vermont.

Mr. Obama’s campaign manager, David Plouffe, set a high bar for Mrs. Clinton, writing in a memo to reporters that without “healthy double digit margins” of victory in Ohio and Texas, she will be “facing almost impossible odds to reverse the delegate math.”

For her part, Mrs. Clinton tried to project a sense that the campaign was far from over. “I’m just getting warmed up,” she told reporters, according to the Associated Press. She said she expected to do well today and was looking ahead to the primary in Pennsylvania on April 22 and the states that come next.

But at another point, she appeared less certain that the race would continue beyond today. “I intend to do as well as I can on Tuesday, and we’ll see what happens after that,” Mrs. Clinton said, the AP reported.

In a conference call yesterday, two top Clinton advisers suggested that popular-vote victories in Texas and Ohio would give her a clear argument for pressing forward by virtue of her wins in many of the nation’s largest states, including New York, California, and New Jersey.

What could complicate matters is if Mrs. Clinton won the popular vote in Texas but Mr. Obama takes more delegates — a distinct possibility given the unorthodox nature of the two-part election, in which some of the delegates are awarded in a traditional primary and the rest are allocated through separate caucuses tonight.

Mr. Obama leads Mrs. Clinton by 110 delegates in the race to 2,025, according to an AP tally, but the New York senator will have a difficult time making up that gap with only narrow victories, because of party rules that allocate delegates proportionally. Hundreds of superdelegates are still up for grabs, but those unpledged party leaders have trended toward Mr. Obama in recent weeks, and for Mrs. Clinton to stake her candidacy on superdelegates would undoubtedly mean a brokered convention that many Democratic elders are intent on avoiding.

That leaves a difficult decision for Mrs. Clinton if today’s results, whether because of a split of Ohio and Texas or an insignificant delegate swing, are inconclusive.

One scenario unlikely to occur, short of a blowout by Mr. Obama, is an immediate exit by Mrs. Clinton tomorrow, a professor of public affairs at Baruch College, Douglas Muzzio, said. “I would be dumbfounded,” he said, adding that he expects the Clintons to take time to closely examine their political options.

Amid the speculation over Mrs. Clinton yesterday, the candidates stumped across Ohio and Texas in a heated final day of campaigning. Mrs. Clinton released a television ad explicitly criticizing Mr. Obama for failing to hold any hearings on Afghanistan in his year-long tenure as chairman of a Senate subcommittee with jurisdiction over NATO. The Obama campaign, meanwhile, struggled to explain the disclosure that a senior economic adviser, Austan Goolsbee, had met with Canadian officials and discussed Mr. Obama’s position on the North American Free Trade Agreement.

Campaign officials had previously denied that the meeting took place after a Canadian news outlet reported that Mr. Goolsbee had told officials that Mr. Obama’s rhetoric criticizing Nafta did not reflect his true intentions.

Mr. Plouffe acknowledged the meeting yesterday but said it was “not a formal conversation” and that the issue had been “way overblown.” The Canadian Embassy took the rare step of issuing a statement expressing regret over the incident and pledging not to interfere in American electoral politics. Mrs. Clinton also weighed in yesterday, saying the incident raised “serious questions” about Mr. Obama’s campaign promises.