A successful deal must boost farm income by providing new export opportunities that are "comparable in magnitude" to any farm subsidy cuts the United States would have to make, Senate Agriculture Committee Chairman Tom Harkin, an Iowa Democrat, and 16 other panel members said in a letter.

"Anything less will not receive our support. ... If you are presented with an unbalanced text, we urge you to reject it in favor of continued negotiations," the bipartisan group wrote to U.S. Trade Representative Susan Schwab.

The letter came just before a high-stakes meeting in Geneva to try to reach a breakthrough in nearly seven-year-old world trade talks. Although negotiators weren't expected to reach a final deal next week, they are being asked to agree on formulas for cutting farm subsidies and tariffs that were originally supposed to be decided in 2003.

If they don't achieve that, many believe the Doha round trade talks will go into the deep freeze for years.

Any deal reached by the White House must be approved by Congress, forcing U.S. negotiators to pay close attention to farm state lawmaker demands.

Earlier this week, farm groups sent a similar letter to President George W. Bush complaining that current proposals ask them to give up more in domestic farm subsidies that they can expect to gain from new export opportunities.

INDIA PUSHING DEEP U.S. CUTS

The proposed text requires the United States to cut its spending cap on "overall trade-distorting support" to a range between $13 billion and $16.4 billion, from the current ceiling of $48.2 billion, which is well above what's actually spent.

India and other developing countries have urged the United States to cap spending closer to $7 billion. That is roughly the size of current U.S. outlays, which have fallen in response to high international commodity prices.

"We must emphasize that U.S. commitments on domestic support cannot be based on an assumption that the current high levels of commodity prices will continue in perpetuity; the historical evidence strongly suggests the contrary," the farm groups said in their letter to Bush.

Meanwhile, U.S. business groups are nervously watching talks on manufactured goods, where leading developing countries are balking at demands to open their markets to more imports.

"We want Doha to succeed, but we won't support a bad deal," Christopher Wenk, senior director for international policy at the U.S. Chamber of Commerce, told reporters on Friday.

The United States already has low tariffs on most manufactured goods, so proposed cuts would not cause pain for most manufacturers. But high tariffs in sectors such as textiles, clothing and autos would be brought down sharply.

The U.S. Chamber and other business groups don't want the United States to give up those negotiating chits unless developing countries offer significant new market openings, and so far they're not, Wenk said.

A third set of negotiations to liberalize international trade in services is in even worse shape, so next week's meeting will be key to showing how much developing countries are willing to up their markets, Wenk added.

(Editing by Eric Beech)