After years of rivalry and poaching each other's members, the nation's two largest teacher unions are weighing a merger that would create a powerful new force in organized labor.

But thousands of teachers must approve the marriage between the National Education Association and the American Federation of Teachers. The crucial test comes July 5, when more than 10,000 delegates to the NEA's annual convention cast ballots.The outcome is uncertain. Two-thirds of the NEA delegates must approve the merger, and some large state affiliates are opposed. The 106-53 vote by the union's executive board is exactly two-thirds, but delegates vote secretly and independently.

NEA president Bob Chase remains confident but says there is a lot of persuading to do in less than a month.

"Although there are some who are against it, there's absolutely no reason why we can't get the two-thirds vote," Chase said.

Union leaders say the merger is needed to rally forces in the face of eroding confidence in public education and legislative attempts to channel tax money into private schools through vouchers or tax breaks.

Both groups are stressing a "new unionism" that they say puts academic quality and school improvement on the same level as pay, benefits and job security. They have been working jointly on teacher quality, construction, discipline and safety.

But opponents and undecideds worry that the much-larger NEA, with 2.4 million members - mostly classroom teachers - will sacrifice too much of its democratic structure and professional identity in a merger with the AFT.

Smaller, with about 950,000 members, the AFT belongs to "Big Labor," the umbrella group known as the AFL-CIO.

Advocates say those ties can help. The 3,200-member Toledo Federation of Teachers, part of the AFT, credits support from the United Auto Workers for helping push the school board to settle on a contract this year and avert a strike.

On a larger scale, the new organization could reach more than 13 million union households to rally support for its issues.

But opponents worry that the NEA will be distracted by other issues if it forms ties with the AFL-CIO.

The NEA began in 1857 but did not emerge as a union that did collective bargaining until the 1960s.