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.

"Whether your resistance to their current power grabs collapses or not, they will work just as hard to defeat you next time. But if you are seen to cave in now, next time they will be stronger," they write. "And if Republicans cave in now, when it really counts, next time you will be weaker, because your conservative base will be outraged. Many who worked hard to elect you in the past will never lift a finger for you again."

The letter brings together leaders of tea party groups, like Ginni Thomas -- wife of Supreme Court Justice Clarence Thomas -- as well as free-market advocates and a wide variety of social conservatives, including Phyllis Schalfly, president of the Eagle Forum, and Gary Bauer, president of American Values.

They represent long-time conservative voices, like Ed Meese, the one-time Reagan official, and the new voices of the conservative blogosphere, including Erick Erickson, editor of the popular site RedState. And, demonstrating their power to make good on threats to mount primary challengers to Republicans who go against party ideals, their numbers include big money donors, notably Foster Friess, the Wyoming businessman who was the leading funder of a super PAC that supported Rick Santorum.

Though the letter appears motivated by the current talks about the fiscal cliff, it makes a broader case that Republicans not concede ground to Democrats, despite the outcome of the November election. Resisting Obama's agenda, they argue, will result in "most Americans will become fed up with the left's failed agenda and be ready for a change." But working with Democrats now, they warn, will result in a conservative uprising.

"If such deals are made, conservative organizations and conservative and liberty-loving voters would see that the current leadership is not an acceptable alternative to the left. Conservatives would then likely repeat what they did in the 1970s, when they systematically and successfully undertook a multi-year effort to replace Republican congressional leadership."

Meanwhile, the co-directors of the liberal Campaign for America's future Thursday warned Congressional Democrats not to give on key entitlement cuts for a deal, particularly on adopting a less generous definition of inflation that would result in a cut in Social Security cost of living increases. 

“No deal is a far better alternative than a bad deal — and the grand bargain now being discussed is a very bad deal," said Robert Borosage, one of the group's co-directors, referring to the possibility of a deal that pairs tax increases with entitlement cuts.

The new pressure illustrates the difficulties Congress and Obama are facing in finding a way out of the fiscal cliff stalemate, even as most voters in both parties want a compromise. Obama has insisted there can be no deal without an agreement to raise tax rates on the wealthy. A number of centrist Republicans have argued the party should accept those terms but demand significant spending and entitlement cuts in return. Still, many conservatives insist a tax hike must remain off the table, making a deal an increasingly unlikely possibility. Meanwhile, liberals are urging Obama to resist the changes in Medicare and Social Security that Republicans interested in that kind of deal would demand in return.