Amid concerns that any changes to the House-approved “cash for clunkers” reauthorization could cause delays, U.S. Sen. Tom Harkin said he is justified in  pushing an amendment to the additional $2 billion influx program.

Harkin proposes limiting the $4,500 rebate program aimed at getting “gas guzzling” cars off the street to individuals with an adjusted yearly gross income below $50,000 or couples/families with an income below $75,000. He realizes that changing the bill already passed by the now-recessed House may create delays since the bill would need to go to that chamber for re-approval, but Harkin said he is “conscience-bound” to attempt the change.

“[A delay] wouldn’t be a tragedy,” Harkin said Thursday morning during a conference call with reporters. “There are reports of dealers who have developed a shortage of certain well-selling cars under the program, but there are still plenty of other cars out there.”

In particular, Harkin, indicating that four out of five cars offered under the program are foreign, said he believed there were many more American-made vehicles that are fuel-efficient and should be a part of the program. A U.S. Department of Transportation noted that the Toyota Corolla was the car most often purchased under the program. It was followed by the Ford Focus, Honda Civic, Toyota Prius and Toyota Camry.

“[The] last time I was rushed to judgement on something like this was last year on the TARP program, and I’m not going to get rushed into a ’sky-is-falling’ scenario again,” he said. “Just because the House passed it and they went home, we can’t change it? Nonsense.”

Despite his bravado on the subject, however, Harkin indicated he did not expect that his amendment would win approval. Most politicians, he said, are ready to pass the House version tonight as scheduled and begin their own August recess.

As Mike Lillis of The Washington Independent reported this morning, the additional $2 billion proposed for the popular clunkers bill isn’t just being pulled from thin air.

…The House last Friday provided the generous lifeline to the wildly popular clunkers program — which grants drivers up to $4,500 to scrap their gas guzzlers for more fuel efficient vehicles — and the Senate is poised to pass that bill Thursday. But there’s a glitch. The proposal steals its funding from a Department of Energy program encouraging the development of renewable energy technologies. That initiative, granted $6 billion under this year’s stimulus bill, provides federal loan guarantees to clean energy projects — including solar, wind and biofuel innovations — in hopes of spurring private investment in those industries. Tens of billions of dollars in loan applications are before the DOE, but the program funding was seen by lawmakers as low-hanging fruit because it wouldn’t be spent until next year, at the earliest.

The saga has created a dilemma for a number of lawmakers who support the cash for clunkers extension but don’t want to pilfer from the loan guarantee program to fund it. “I would hate to see us take money from that source,” Sen. Jeff Bingaman (D-N.M.), who chairs the Senate Energy and Natural Resources Committee, told CNBC on Tuesday. “I hope we can find an alternative.”

They didn’t. Although seven amendments to the House proposal will be offered on the Senate floor Thursday afternoon, none aims to locate a new source of the $2 billion…

The program has also earned criticism from former Republican presidential hopeful Ron Paul, who believes the program hurts the poor.