Conferees working toward agreement on the fiscal 2013 defense policy bill are locked in negotiations that most likely will lead to the inclusion of language requiring the Defense Department to conduct a study outlining a plan for building an East Coast missile defense site.

But particularly thorny negotiations revolve around a related provision from the House bill (HR 4310) that would mandate the site’s construction, something opposed by Democrats, particularly Senate Armed Services Chairman Carl Levin of Michigan.

Republicans are trying to include language in the bill that reflects their strong desire for construction of the East Coast site. In the end, any agreement may boil down to directing the Pentagon to conduct an environmental-impact study and then developing a plan for consideration.

Such a development would be a victory for Republicans, given that their proposal for the site virtually came out of thin air at the beginning of this year. At the time, it was viewed by senior Republican aides as the beginning of a long campaign to sway lawmakers to their side.

The proposal reflected growing fears that Iran was developing nuclear weapons and a means to deliver them to European and, ultimately, U.S. targets. Currently, the United States employs ballistic-missile Aegis destroyers to provide limited defensive capability while building toward missile defense land-based sites in Europe.

Several conservative defense experts had discussed and written about the idea of an East Coast base before this year, but the Missile Defense Agency, the Pentagon arm that oversees missile defense, has never called for such a site.

Congressional aides were reluctant to say more than the issue and the negotiations are “complex.”

“We’re still working on that,” one senior congressional aide said of the final language on the provision. “We’re working something out.”

The comment reflects the delicate nature of negotiations on the entire conference report, which must be completed on a truncated timeline.

One congressional aide said that conferees from both parties and chambers must stick together on the talks. The fear is that any premature disclosure about particular provisions could undermine the entire bill, throwing its completion into doubt.

Levin said the final bill most likely would be filed in the House and Senate by Tuesday, leaving enough procedural time for lawmakers in the House to adopt the conference report as early as Wednesday and for those in the Senate to clear it by Friday at the latest.

“I think we are hoping to get to full conference by Tuesday,” Levin said. “Their conferees were appointed [Dec. 13] so that the conference report could get to the House floor, I hope, on Wednesday.”

Clearing the conference report would continue more than a half-century of success in completing the national defense authorization bill.

The House version of the defense policy bill, which was passed in May, included a provision sponsored by Rep. Michael R. Turner, R-Ohio, the House Armed Services Strategic Forces Subcommittee chairman, that would mandate an anti-ballistic-missile site on the East Coast.

The Senate version of the bill (S 3254) did not include such a provision, although Kelly Ayotte, R-N.H., and Joseph I. Lieberman, I-Conn., had entertained the idea of offering an amendment that would have called for a study into the matter. They later decided not to offer the amendment, in an effort to expedite passage on the Senate floor.

Turner’s amendment was later bolstered when a congressionally mandated report on missile defense by the National Academy of Sciences, ordered by Congress in the fiscal 2009 defense authorization bill (PL 110-417), validated some GOP concerns that the president’s European phased missile defense plan would not provide enough of a defense of the homeland against a limited ballistic-missile attack from Iran.

The report noted that an East Coast site “obviates the need for early interceptor bases” in Europe, unless they are required for European defense.

A senior Senate Democrat, however, argued that there has been no military recommendation or requirement offered to support an East Coast anti-missile battery.

Nonetheless, conferees and staff are working to fashion language about the missile site that is palatable to Democrats and avoids a presidential veto yet also reflects the will of the House and the desire of many in the GOP that the Pentagon move toward an East Coast missile defense site.

Ayotte said she believed that conferees could find a way to include a provision supporting an East Coast site with “some conditionality.” But, she emphasized, it would be important for the Defense Department to look at the environmental impact of such a site.

Even though progress on the issue has been made, one senior aide noted that it won’t be complete until Tuesday, when conferees sign off on the conference report.