Senator Obama is saying decisively that he does not support private school vouchers, while sticking with his support for incentive pay for teachers based on their students' performance.

Click Image to Enlarge Chip Somodevilla/Getty Senator Obama greets supporters after addressing a town hall meeting at the gymnasium at Robinson Secondary School July 10, 2008 at Fairfax County, Virginia.

Mr. Obama made the remarks yesterday in a telecast speech to the American Federation of Teachers' national convention in Chicago after the union voted overwhelmingly to endorse his presidential bid.

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The convention will also be the site today of the long-anticipated accession of the New York City union president, Randi Weingarten, to the presidency of the AFT, the national branch of the local United Federation of Teachers. Ms. Weingarten, who was the only candidate nominated for the national job after the former president announced he would retire, is planning to hold on to her position as UFT president while she steers the national union.

Today will mark her first day as president of both unions. It will be closely watched, both for any sign that Ms. Weingarten might be taking her eye off the ball in New York, where she is a formidable figure both on education policy and in state and city politics, and for signals of what role Ms. Weingarten will play on the national stage.

Ms. Weingarten's parents have traveled to Chicago to watch the speech she will deliver this morning.

Both Mr. Obama and Ms. Weingarten have aroused a mix of excitement from those who push for extensive change in public schools and skepticism from traditional union members who oppose the so-called reform policies, such as charter schools and plans to tie teacher pay to student test scores.

In February, Mr. Obama set off concern among some union members when he told a Milwaukee newspaper's editorial board that he was open to supporting private school vouchers if research showed they work.

His campaign quickly issued a statement that said, "Senator Obama has always been a critic of vouchers," and Mr. Obama echoed that sentiment yesterday, saying that while he supports charter schools, he opposes private school vouchers.

"We need to focus on fixing and improving our public schools; not throwing our hands up and walking away from them," he said.

Mr. Obama also raised concerns when he endorsed the idea of "merit pay" at a convention last year for the other national teachers union, the National Education Association.

In his address to the NEA this year, he acknowledged that the idea "wasn't necessarily the most popular part of my speech last year," but vowed to stand by it, eliciting some boos.

He also stood by the idea in his speech to the AFT convention yesterday, which he made via satellite from San Diego.

"When our educators succeed, I won't just talk about how great they are; I will reward them for it," Mr. Obama said. He listed several cases in which districts could give teachers a salary increase, including if they serve as mentors; if they learn new skills, and if they "consistently excel in the classroom."

Those at the AFT convention said that no boos followed the remarks, though some union members later said they were concerned by them.

"That was the one statement that raised our eyebrows," the president of the AFT's Los Angeles chapter, A.J. Duffy, said yesterday. "Our question is what does that mean, 'who consistently do well in classrooms,' and based upon whose guidelines? Is it a principal, a test score? We're going to continue to have dialogue with him."

Other AFT members said they were offended that Mr. Obama did not make his speech live in Chicago, his hometown. In a change from the practice of some previous Democratic presidential candidates, Mr. Obama also delivered his remarks to the NEA via satellite.

The executive director of the lobbying group Democrats for Education Reform, Joseph Williams, characterized that decision as a "Velvet Snub" indicative of Mr. Obama's willingness not to become owned by special interest groups, addressing them as if across a protective velvet rope.

In her tenure as New York City union president, Ms. Weingarten's work starting two charter schools and her collaboration with the Bloomberg administration on a performance-based pay plan have won her a similar mix of skeptics and boosters.

Mr. Duffy said some Los Angeles teachers are concerned about Ms. Weingarten, though he supports her.

A New York City teacher who is attending the Chicago convention and is a member of a group that has opposed some of Ms. Weingarten's ideas, Lisa North, said that in Chicago she is hearing from many union members across the country who are skeptical about policies such as charter schools.

"There are AFT members that would like to see the AFT helping organize teachers to fight the privatization, the high-stakes testing, and No Child Left Behind," Ms. North said.

Reg Weaver, the president of the NEA, which is seen as more stringent than the AFT in its opposition to such policies, would not say whether there are any differences between his positions and Ms. Weingarten's.

He added: "I think she's good, has been good, and will be great."