As Speaker Nancy Pelosi and other leading congressional Democrats begin to talk about amending or altering the Medicare prescription drug bill enacted in 2003, the Bush Administration is making it clear that it will stand firm against any effort to change the measure President Bush considers one of the proudest accomplishments of his stint in the White House.

“If Democrats tinker with it [the prescription drug bill], and in effect destroy market incentive, the President will veto these attempts,” a top administration official told me this afternoon.

The same source, who requested anonymity, pointed to figures that the measure -- the largest entitlement since Medicare was enacted in 1965 -- has actually brought down the cost of drugs, the estimated premium cost dropping from $37 to $25 since the President signed the measure three years ago. “Market works,” he said, and the President -- who worked members of the House tirelessly until the bill was passed by two votes -- will “do everything to protect markets.”

The source’s promise went a step further than White House Press Secretary Tony Snow did on Friday when I asked about possible vetos of measures passed by the Democratic Congress. As Snow told me, “I’m not going to get in a position up here of issuing veto threats of bills that haven’t been filed. It’s irresponsible. It’s great for picking a fight, but you know, the point we’ve made with this Congress is we’re looking forward to working with this Congress.”

A day before, however, Snow’s deputy, Tony Fratto, filled in for him went a step further and hinted at a veto when I asked about reports congressional Democrats were considering amending the prescription drug bill with regard to forcing drug companies to lower the cost of drugs to Medicare beneficiaries.

“Some of the proposals being discussed seem to have a bit of solution in search of a problem,” Fratto told me. “The supposed problem is that prescription drugs are not being negotiated low enough, given market forces. And the evidence that we see is that the private insurers who are negotiating with drug companies are significantly lowering prices far beyond what any experts and analysts had anticipated.”

Fratto went on to say: “We have to be very careful about tinkering with programs that have that level of success and this level of popularity with beneficiaries. And for a brand new program, that’s really impressive. And so we’d want be careful before reforming any of these programs.”

That’s almost drawing a line in the sand. Today, the Administration went a step further: If Congress tinkers with the prescription drug bill, Bush will cast the second veto of his presidency.