If you want to know why there is so little support among House Republicans for the Senate immigration bill, look no further than the letter sent by three Republican activists to House Speaker John Boehner, R-Ohio, Tuesday.

In the letter, American Action Forum President Douglas Holtz-Eakin, American Conservative Union Chairman Al Cardenas, and Americans for Tax Reform President Grover Norquist do say that they consider the Senate bill to be “progress” on the immigration issue. But they then fail to even mention, let alone defend, the core provisions of the legislation.

President Obama has made it crystal clear he will not sign any immigration bill unless it contains two key elements: 1) immediate legalization for the 11 million illegal immigrants currently in the United States today; and 2) a guaranteed path to citizenship for those legalized immigrants.

The Holtz-Eakin/Cardenas/Norquist letter does not address either of these issues.

The letter does call for “a tough but humane process to earned legal status for those undocumented immigrants who wish to stay in the United States,” but at no point does the letter contain the word “citizenship.”

The letter also makes no mention of the timing of the legalization process. Many House Republicans have repeatedly said they would be open to legalizing those currently in the country illegally, but only after various border security measures have been implemented. At no point does the letter make the case why legalization must come before border security, as it does in the Senate immigration bill.

Until pro-reform Republicans can defend the core elements of the Senate immigration bill, do not expect it to make any progress in the House.