WASHINGTON (AP) -- The White House is in serious discussions over legislation long opposed by many Republicans to guarantee that insurance for mental health disorders is as comprehensive as that offered for other illnesses.

A White House official said Thursday that President Bush may endorse the idea of "mental health parity" at an event Monday in New Mexico, home of Republican Sen. Pete Domenici, who has championed the cause for many years.

The official, speaking on condition of anonymity, cautioned that no deal has been reached yet on the legislation, which is sharply opposed by business groups, which fear that it will significantly increase the cost of health insurance.

Politically, the legislation would bolster the president's "compassionate conservative" credentials.

"The signs are good from the White House and we feel very optimistic," said Allison Dobson, spokeswoman for Sen. Paul Wellstone, D-Minn., who has worked alongside Domenici on the issue.

At issue is the scope of the law. A limited version of parity, which expired last year, barred employers from setting lower annual or lifetime limits on coverage for mental illnesses than they set for other illnesses, which had been a common practice.

But the law did not prevent them from requiring higher co-payments or deductibles for visits to psychiatrists.

Last year, the Senate added the mental health parity requirement to a spending bill. The provision barred businesses with more than 50 workers from setting either higher co-payments or higher annual and lifetime limits for mental illnesses. But it was opposed by key House Republicans and removed in negotiations over the final bill.

The president supports the broader parity, according to White House and congressional aides. At issue is whether the parity will apply to all diagnosed mental illnesses, as the Senate bill provided, or only major diseases.

Also at issue is whether businesses could opt out if costs rise too much. The original law that expired last year allowed them to skirt the new requirements if they could show that they raised health care costs by more than 1 percent.

In this case, the nonpartisan Congressional Budget Office (news - web sites) has estimated that the full Senate parity bill would raise costs by just under 1 percent.

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