With a landmark climate change bill expected to be unveiled today, Senate Republicans are already poking at differences within the Democratic ranks in an effort to put some constraints on the legislation.

Republicans kicked off their effort last week by offering amendments to a spending bill to restrict the administration’s ability to regulate carbon emissions without Congressional action — playing up concerns among Republicans and some Democrats that the Environmental Protection Agency would do an end-run around Congress.

“The administration has chosen to use the back door, which is issuing regulations [because] they can’t get the Waxman-Markey bill through the front door,— Republican Policy Committee Chairman John Thune (S.D.) charged on the floor last week.

The host of amendments offered to the Interior appropriations bill, including one from Republican Conference Vice Chairwoman Lisa Murkowski (Alaska) that would have banned the EPA from regulating carbon dioxide, came as Senate Democrats prepared to unveil a massive climate change bill this week.

Murkowski reached out to moderate Democrats to support her amendment, but Democratic leaders actively whipped against the proposal, which never came up for a vote.

“I think many of the people I spoke with shared my view that we should not let some unelected administration officials decide regulations,— Murkowski said of her discussions with Members on both sides of the aisle.

Asked why her amendment was never considered for a vote, Murkowski suggested, “There was a great deal of concern by the Democratic leaders that this would send a bad signal to the Obama administration. They were worried about the outcome so they pulled their Members back.—

A Democratic leadership aide said it was clear their side had the votes to beat Murkowski’s amendment.

But on Tuesday — the day before Environment and Public Works Chairman Barbara Boxer (D-Calif.) and Foreign Relations Chairman John Kerry (D-Mass.) are expected to unveil a climate change bill — moderate Democrats still appeared squishy on the issue.

“I am not committed to [carbon] cap-and-trade under any circumstance,— Sen. Mary Landrieu (D-La.) said.

“It’s a difficult issue,— added Sen. Evan Bayh (D-Ind.), who hails from a manufacturing state.

But moderates from Midwest and coal states have largely stayed mum on the issue.

Many fear a Boxer-drafted climate change bill would seriously harm the manufacturing industry and thus be politically radioactive in a year in which moderates already face a difficult vote on health care.

“The climate change debate is being driven by California and Massachusetts,— Murkowski spokesman Rob Dylan said. “People forget what life is in the middle of the country, and I think that’s what we’re trying to talk about.—

The House narrowly passed its version of climate change legislation in June, a controversial cap-and-trade plan that would require steady reductions in carbon dioxide emissions and would set a national standard for renewable electricity. But that plan lacks sufficient support in the Senate, where 60 votes are needed to avert a filibuster.

Despite today’s bill introduction by Boxer and Kerry, and the markup expected to occur in the Environment and Public Works Committee next month, the issue of climate change has rapidly moved down the Democratic priority list as leaders remain focused on passing a health care reform bill by the end of the year.

Emily Pierce contributed to this report.