A group of big right-wing activists wrote an open letter to House and Senate Republicans, telling them they are "entering into a period of testing," wherein their conservative principles will be challenged and "the whole leftist apparatus is gearing up to panic you and to force you to cave in. Don’t do it."

The letter, posted by conservative columnist Morton Blackwell on RedState.com, urges lawmakers to be strong against the liberals' "current power grabs."

"If Republicans cave in now, when it really counts, next time you will be weaker, because your conservative base will be outraged," the letter says. "Many who worked hard to elect you in the past will never lift a finger for you again."

The letter concludes:

So it’s in the interest of the country and in your personal interest for you to use the power you unquestionably have now to stand firm and not surrender your conservative principles, no matter how loud the clamor of people whose central interest is to advance the left’s agenda.

Blackwell is president of The Leadership Institute, the group that helped launch James O'Keefe's career. And he's in good company: The list of signatories reads like a who's who of right-wing activism. For starters you've got Colin Hanna of the Tea Party group Let Freedom Ring, Phyllis Schlafly, president of the Eagle Forum, and Erick Erickson, CNN contributor and RedState's most well-known blogger.

There's also Brent Bozell, head of the Media Research Center, who once said "you might want to say" President Obama looks like "a skinny, ghetto crackhead"; Liberty Council chair Mat Staver, who has accused the Obama Administration of running an “affirmative action” program to put Muslims into top national security positions so that the Muslim Brotherhood can infiltrate the government; and birther extraordinaire Joseph Farah of World Net Daily.

Here's the letter (the full list of signatories is here):

An Open Letter to U.S. House and Senate Republicans

Dear Republican U.S. Representatives and Senators:

You are entering into a period of testing.

In the House, the nation elected in 2012 one of the largest Republican majorities in the past 100 years. You have a mandate to fight for conservative principles that is arguably much broader than the one that narrowly reelected President Barack Obama claims to have for his leftist agenda.

Of course, House Republicans alone cannot pass a law, but united you can stop any bill which violates the principles you publicly committed to support.

In the Senate, you have more than enough Republicans to prevent the passage of anything truly harmful to our country.

Nevertheless, you are already being pressured to abandon the principles voters elected you to uphold.

The major news media, Democrat politicians, liberal interest groups, and even some weak-kneed Republicans are demanding that you not use the powers given to you by the electorate to stop the power-grab agendas of President Obama, Senate Majority Leader Harry Reid, and House Minority Leader Nancy Pelosi.

The Democrat leadership does want to “fundamentally change” America.

They want to increase federal power, federal spending, federal taxes, federal deficits, and federal debt. They want to give more advantages to their liberal allies and put more federal money into the pockets of their political cronies.

They want to undermine further traditional values and to surrender more of our national sovereignty to international bodies.

They want to weaken our country militarily and move America more rapidly in the direction of European socialism.

Their entire leftist agenda opposes the principles on which you ran and the principles of the millions of people who elected Republicans to Congress.

And now we count on you to stop them. Nobody else can stop them but you. You have the power, if you will use it.

The whole leftist apparatus is gearing up to panic you and to force you to cave in. Don’t do it.

All of them, the liberal Democrats, the labor unions, the leftist ideologues in the so-called “progressive” movement, and their allies in the print, broadcast, and online media will try to convince you that your political careers will be over if you don’t cave in.

Let us remind you that these same people have always tried to defeat you — yet you won election.

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.

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.

There are two ways President Obama, Nancy Pelosi, and Harry Reid could get what they want out of the Congress now.

First, just enough Republicans could vote with the Democrats and against most other Republicans to pass what Obama, Pelosi, and Reid want passed.

Second, the Republican leadership could negotiate a “deal” with the Democrats and take most Republican Representatives and Senators with them.

Either of those two courses would be a disaster for conservative principles because they would result in permanent advances for the “fundamental changes” the left wants to impose on our country.

Let us also remind you that a great many potent conservative organizations and millions of conservative and liberty-loving voters do not believe in the divine right of incumbents to be re-nominated. Conservatives know how to recruit and support candidates.

In recent election cycles the left has virtually eliminated “moderate” Democrats from the Congress. And you have no conservative Democrat colleagues at all.

At the same time, those election cycles have resulted in the nomination and election of fewer and fewer Republicans who play footsie with liberal Democrats.

In the Republican Party, conservatives dominate the primaries more and more, a process almost certain to continue.

Not only do you now have one of the biggest House Republican majorities in the past century, in the Senate, you have 45 Republicans, more than enough to stop many bad decisions there.

Suppose just enough Republicans cross the aisle to provide Democrats a majority in vitally important legislative battles now upon us. They will find themselves, come election time, the targets of both liberal Democrats and conservative Republicans, which isn’t likely to be healthy for their political careers.

Suppose, on the other hand, that the congressional Republican leadership negotiates a “deal,” a “compromise,” with Obama, Pelosi, and Reid.

The historic record of such deals shows that the Democrats usually clean the Republicans’ clocks.

You will remember that, with all good will, President Reagan and then the first President Bush made deals to support some tax increases in return for Democrat promises to cut spending even more. In both cases, the new taxes were quite real, but far from cuts in spending, spending increases followed.

Right now it is generally but incorrectly believed that Republicans aren’t able to negotiate from a position of strength.

Of course, this time every element of the left will be in full-throated roar, yelling that the country will collapse if Republicans don’t give in to their demands.

There is some danger that the congressional Republican leaders will decide to accept whatever deals they can get, which might mean that Democrats get real gains and Republicans again get empty promises. Or Democrats would get permanent changes and Republicans would get changes easily reversed by a future Congress.

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.

With President Obama in office for another four years and his leftist appointees working tirelessly to implement the left’s agenda, there is a virtual certainty that the economy will not have a significant recovery. More likely the country will suffer a continuation of Obama’s permanent recession.

Yes, this is a time of testing for you.

If Republicans provide a consistent, conservative alternative, the time will come, probably sooner than later, when most Americans will become fed up with the left’s failed agenda and be ready for a change to “throw the bums out” and restore limited, Constitutional government.

So it’s in the interest of the country and in your personal interest for you to use the power you unquestionably have now to stand firm and not surrender your conservative principles, no matter how loud the clamor of people whose central interest is to advance the left’s agenda.