President Barack Obama this week is set to become the central player in Senate Democratic leaders’ push to enact health care reform by the end of the year, a role the administration has been preparing for over the past several weeks.

Senior White House officials are scheduled to be in the room throughout negotiations to merge competing Senate health care bills from the Finance and Health, Education, Labor and Pensions committees, with the expectation that they will make key decisions to mediate disagreements. In advance of the floor action to follow, Obama and top administration officials have been lobbying Senate Democrats to secure support for a final package.

“The White House presence in the merger will be huge, and it has to be,— a senior Democratic Senate aide said Monday. “President Obama will have to weigh in on the most difficult issues.—

The merger negotiations are set to begin later this week under the direction of Senate Majority Leader Harry Reid (D-Nev.), with Finance Chairman Max Baucus (D-Mont.) advocating on behalf of his committee’s bill and Sen. Chris Dodd (D-Conn.) pushing to preserve the HELP legislation. The negotiations are scheduled to take place in Reid’s Capitol office, just off the Senate floor.

White House Chief of Staff Rahm Emanuel and Nancy Ann DeParle, Obama’s chief health care adviser, are expected to be at the table throughout the talks. White House Office of Management and Budget Director Peter Orszag also is set to play a role, primarily on issues where health care and the federal budget intersect. Staff from the administration, leadership and the committees began joint preparations for the merger of the bills about a week ago.

Although Obama’s personal lobbying of centrist Democrats has received the most attention, the president has actually spent considerable time appealing to Senate liberals, as well. DeParle has been visible on Capitol Hill for weeks to assist in that capacity, and Democratic sources say Obama is going to have to make the final call on the controversial issues, including whether to push for the public insurance option.

With negotiations over the public insurance option in a delicate phase, the White House is mostly avoiding public comment on the matter. At press time, administration officials had not responded to request for comment.

In fact, in his last two health care-related remarks — an appearance in the Rose Garden with doctors Monday and his weekly radio and Internet address released Saturday — Obama did not mention the public option.

At the daily briefing Monday, White House Press Secretary Robert Gibbs was asked whether Obama was “still working— for the Senate to pass a public option. Gibbs skirted the question.

“We’re still working to make sure that we have choice and competition in this legislation,— he said.

Sen. Mark Begich (D-Alaska) said the Obama administration has maintained a noticeable presence in the Senate over the past eight months as top White House officials kept an eye on the progress of the effort to bring a health care reform bill to the floor. Still, Begich observed that the administration has generally trod lightly as the Senate worked through the process of drafting and marking up the Finance and HELP bills.

Begich has not spoken to Obama since the president convened a meeting of Senate Democratic moderates at the White House the day after his Sept. 9 speech to a joint session of Congress. But Begich said he expects the administration to begin throwing its weight around more forcefully as the process moves from Finance Committee markup to the merger and into the floor debate.

With a vote to report the Finance bill out of committee imminent and the merger negotiations to settle on a single floor vehicle about to commence, Begich also signaled that he and other Democratic freshmen are set to begin pushing for their health care reform priorities. Begich said these freshmen would be putting ideas “on the table.—

“They’ve been regular. But I think it’s not been — I wouldn’t call it an aggressive— involvement, Begich said of the Obama administration’s presence in the Senate on the health care front, adding that he expects that to change now that the Finance Committee has marked up its bill.

“I think now that that’s starting to tip to be done, I think we’ll see more,— Begich said.