By Susan Cornwell

WASHINGTON (Reuters) - American knowledge ofRussia's nuclear capabilities will dwindle if a new nucleararms treaty with Moscow is not ratified, the chief U.S.negotiator argued as a Senate panel Friday scheduled a voteon the document.

The new START treaty with Moscow is one of the centralplanks of President Obama's nuclear policy, and part of hiseffort to "reset" relations with Moscow. Obama wants itratified by the Senate this year. But some Republican supportwill be needed, and so far little has emerged.

Assistant Secretary of State Rose Gottemoeller contendedthat the longer it takes to ratify the new agreement, the lessWashington will understand about Russia's nuclear arsenal.

Inspections of bases inside Russia stopped when the oldSTART treaty expired in December.

"U.S. knowledge of Russian nuclear forces willsubstantially erode over time if the treaty is not ratified andbrought into force, increasing the risk of misunderstandings,mistrust, and worst-case analysis and policymaking,"Gottemoeller wrote in the forthcoming issue of Arms ControlToday.

She was the chief U.S. negotiator on the treaty, whichwould cut the number of nuclear warheads deployed by the UnitedStates and Russia by about 30 percent.

The Senate Foreign Relations Committee on Friday put thetreaty on the agenda for its September 16 meeting. New START isexpected to pass the committee, which has a majority of Obama'sDemocrats as well as the one declared Republican supporter ofthe document so far, Senator Richard Lugar.

But the treaty has to have 67 votes to clear the Senate,meaning it needs at least eight Republican supporters inaddition to the Democrats.

Some Republicans are worried it may limit U.S. missiledefenses, while others want Obama to promise to spend moremoney modernizing the nuclear weapons that remain.

Republican criticism has increased as partisan rhetoricheats up before congressional elections on Nov. 2.

Gottemoeller pointed out that when the first START treatyexpired last December, the United States became unable, for thefirst time in more than 20 years, to conduct nuclear armsinspections inside Russia.

Those inspections dated back to 1988 under the Reagan-eraIntermediate-Range Nuclear Forces Treaty with the old SovietUnion, and had become a vital means of verifying compliance,she said in the article for the September issue of Arms ControlToday. It is the monthly publication of the Arms ControlAssociation in Washington.

The new START provides for up to 18 on-site inspectionsannually, while the old START provided for 28 annualinspections, Gottemoeller acknowledged.

However, there are only half as many Russian nuclearweapons facilities to inspect as before, because some had beenshut down, or were located in Belarus, Kazakhstan, and Ukraine,former Soviet republics that no longer deploy strategicoffensive nuclear arms, Gottemoeller wrote.