By the time President Barack Obama’s $3.6 trillion budget proposal comes up for a vote in the Senate later this week, the high-pitched opposition from Democratic moderates is expected to give way to a chorus of support.

Although the liberals in the Senate Democratic Conference remain concerned that their centrist colleagues might force Obama to accept a watered-down version of his fiscal 2010 spending plan, most predict the president will ultimately get what he wants. Senate Majority Leader Harry Reid (D-Nev.) said as much on Friday.

“There is no split within the Democrats. You’ll see this next week when we vote on this,— Reid said during a roundtable with reporters sponsored by the Christian Science Monitor.

While nonbinding, the budget resolution is considered important because it sets the fiscal and policy ground rules for how much money will be spent next year. Obama’s budget was only slightly adjusted by Senate Budget Chairman Kent Conrad (D-N.D.) and was approved by the budget panel along party lines Thursday.

Several moderate Democratic Senators representing center-right states have claimed the spotlight over the past month as they voiced concern over key portions of Obama’s budget plan, including how to address health care and environmental reforms. They also threatened to withhold support for the spending blueprint, which could prove crucial even though passing the budget resolution requires only a simple majority.

That has left their liberal colleagues quietly seething that they may have to forgo long-sought-after policy objectives despite finally having a president who shares their goals. Although most of the Senate’s liberal Democrats don’t actually believe their Conference’s moderates will derail Obama’s plans, they’re not pleased about the opposition.

“We can stand some difference in opinion. But we cannot stand by and lose the battles,— Sen. Frank Lautenberg (D-N.J.) said. “You have to give the president some latitude and you have to continue to remember that we are a party that believes that we have an obligation to the American people.—

The House this week is expected to easily pass a budget resolution that — like the Senate’s — closely mirrors Obama’s original proposal. Given that Senate Democrats hold 58 seats and need just a simple majority for approval, passage should be forthcoming, even if some of the party’s moderates vote no.

But “no— votes from centrist Democrats are not what the party leadership expects. In fact, Reid and other top Senate Democrats haven’t bothered to whip for votes on the budget.

They’ve left that task to Obama, who was on the Hill last week lunching with his former Democratic colleagues to do just that. Meanwhile, the White House deployed Vice President Joseph Biden to lobby skeptical Members, a move that has proved to be quite effective, Reid revealed on Friday.

Perhaps tellingly, none of the moderates who are lukewarm to elements of the president’s budget used Wednesday’s lunch with him to voice their concerns, according to a Senate Democratic leadership aide.

“While any budget is challenging, this time we have the luxury of a Democratic president to sell this thing and help us round up the votes,— the aide said, adding that neither Reid nor the president are taking the moderates for granted.

Democratic leaders believe the centrists will fall in line at least partly because opposing a popular president from their own party carries political risk. However, some Senate Democratic insiders are crediting Conrad, arguing that the Budget chairman’s revisions to Obama’s plan have made it more palatable.

Conrad’s budget blueprint spends slightly less than Obama’s — and goes further than the president’s to reduce the deficit. Also, Conrad’s proposal does not set aside any money for a health care overhaul or controversial cap-and-trade energy policy.

Obama’s message to the budget skeptics within the Senate Democratic Conference? Get on board or risk losing re-election in 2010.

And in fact — despite the infancy of Obama’s presidency — Democrats worry that their window is narrow to enact program overhauls that have been on their wish list for decades.

“I think President Obama at the caucus [Wednesday] gave an interesting parting shot before he left. He said, Look, we’re all in this together,’— said Sen. Tom Harkin (Iowa), a veteran liberal lawmaker, describing Obama’s message to those Democrats who are running for re-election.

“Those of you who are up in 2010, if we’re still in an economic mess, and we haven’t done health care reform and we haven’t done something on education [and] we haven’t done something on energy — you’re in trouble,— Harkin continued, paraphrasing the president. “I don’t care how far you distance yourself from me, how much you say this is too liberal, too progressive, it ain’t going to help you.—

Questioned Friday about predictions that they are all bark and no bite, some stalwart centrist Democrats pushed back.

Sen. Mark Pryor (Ark.) noted that the Senate’s version of the spending plan emerged from the Budget Committee late on Thursday and that he would have to review it before determining where he stands. Pryor also said the lines of communication between the White House and his fellow moderates have been open.

Moderate Sen. Ben Nelson (Neb.) was even more forceful in disabusing the notion that his concerns on the budget amount to empty rhetoric: “There’s no question in my mind that I’m totally sincere. I’m not bluffing and I’m not playing cards.—

John Stanton contributed to this report.