The leaders of dozens of major conservative organizations joined together Thursday to jointly pen an open letter to congressional Republicans, warning them not to cave to Democratic demands to raise tax rates on the wealthy in a deal to avert the year-end "fiscal cliff" or otherwise compromise on leading conservative issues with Democrats.

Calling the negotiations now underway between House Speaker John A. Boehner and President Obama "a time of testing" for elected Republicans, the group warned that Republicans will harm the movement and the country if they compromise with Democratic demands in a deal to avoid tax increases and mandatory spending cuts at the end of the year, either by allowing Democrats to pass their own plan with a handful of renegade Republican votes or by adopting a broad compromise hatched by Boehner and Obama.