The working alliance between Senate Majority Leader Harry Reid (D-Nev.) and Speaker Nancy Pelosi (D-Calif.) — challenged over the past two years by institutional friction and stylistic differences — has reached a crucible moment.

The completed Democratic takeover is heaping pressure on the two Congressional leaders to deliver on an $800 billion package to save the sputtering economy before racing forward to tackle President-elect Barack Obama’s ambitious domestic agenda. Their ability to do so in the early going of the 111th Congress could exacerbate tensions born over the past two years by uneven margins in the two chambers and often-conflicting expectations.

Spokesmen for Pelosi and Reid said they pursue a common vision with no personal hiccups. But other Democratic insiders said Pelosi was frequently frustrated over the past Congress by Reid’s inability to force intransigent Senate Republicans to heel. Pelosi, in turn, irritated Reid with her stubbornness on policy and short patience for his predicament as a leader who sometimes lacked even a simple majority in a chamber that requires 60 votes to move anything.

It’s hardly a new dynamic in a body where the other party often is the opposition but the other chamber is the enemy. Still, with a once-in-a-generation economic meltdown vexing policymakers and Obama’s hopes for enacting an eye-poppingly expensive rescue plan planted squarely in the Capitol, the stakes could hardly be higher.

There were already indications last week of no clear leader directing the development of the stimulus package. More broadly, the Democratic leaders in the House and Senate offered opposing expectations about how the chambers would function under the new order.

“Clearly, now that there are a lot more Democrats in the Senate, Democrats in the House expect things to move much more quickly there,” said Rep. Chris Van Hollen (D-Md.), Assistant to the Speaker.

But Sen. Byron Dorgan (D-N.D.) said the House should not necessarily expect a smooth ride just because Senate Democrats are likely to end up only one vote shy of a filibuster-proof majority. “I don’t think it will change at all,” said Dorgan, referring to the need for bipartisan support to pass legislation through the Senate.

Democratic lawmakers and strategists nonetheless said Pelosi and Reid, and their respective caucuses, will shelve their egos to enact Obama’s early agenda. “I think that necessity dictates that those tensions — even the natural tensions — will be minimized to the maximum extent possible,” one Democratic Senator said.

Obama may do the most to shake up the inter-chamber dynamic since he now will be the undisputed leader of the party and can act as a “referee” between Congressional leaders, as one senior Democratic strategist said.

Plus, one lawmaker said the desire to get things done under a newly coronated president will likely trump any Reid and Pelosi conflicts, “at least in the short term.” But the Senator added, “Talk to me in six months.”

Pelosi and Reid’s working relationship dates back 20 years to Reid’s first Senate run in 1986. At the time, Pelosi, who had already established herself as a fundraising dynamo in California, was serving as national finance chairwoman of the Democratic Senatorial Campaign Committee. She helped him rake in campaign dollars to finance his winning bid.

But their partnership began to take shape in earnest after the 2004 elections, when they became the top Democrats in their respective chambers. Early on, they teamed up to consult corporate chiefs on how to regain the majority. They agreed to hold back on offering their own agenda and instead focus their fire on an imploding GOP, a strategy that helped them regain control of both chambers in 2006 for the first time in 12 years.

Leading was a trickier prospect. Pelosi managed a Caucus eager to legislate after over a decade in the minority, but Reid had a razor-thin margin to work with.

Pelosi’s desire to protect fiscally conservative Blue Dog Democrats’ demands that both chambers abide by pay-as-you-go budget rules prompted clashes. Time and again, a frustrated Reid held test votes to prove to House Democrats that he did not have the 60 votes necessary to overcome GOP-led filibusters of such things as an alternative minimum tax measure.

Reid also was forced to hold the line on the State Children’s Health Insurance Program bill because the Senate’s bipartisan compromise would have collapsed if it caved to the House’s insistence on adding billions more to the program.

Pelosi allies charge Reid with making pledges, only to break them when faced with the threat of a Republican filibuster.

“Over time, if you can’t keep commitments, there will be lasting damage,” said Rep. Dennis Cardoza (D-Calif.), who represents the Blue Dogs in leadership meetings. “If you see the same [Senate inaction] take place now, you’ll see the House really bow its back on a number of things.

“Once in a while, you’ve got to let them filibuster and call their bluff,” Cardoza added. “It’s not my position to tell the Senate how to do its business, but when time and time and time again, it affects the work we need to do for the American people, it becomes the House’s business.”

Indeed, House Democrats said Reid’s expected pickup of eight Democratic seats — two were still in limbo as of press time Friday — should eliminate his argument that he only has 51 seats to work with.

“The entire House Democratic leadership is salivating for this new dynamic where that Reid crutch doesn’t exist,” one senior Democratic strategist said.

But Senate Democrats insisted that attitude goes to the heart of Reid’s frustrations with Pelosi — her apparent lack of understanding of what it takes to move legislation through the Senate and her unwillingness to pass bipartisan measures out of the House.

“If [Reid] had one criticism of her, it’s that she does try to cram everything through, and she doesn’t reach out on a bipartisan basis,” which makes the process even more difficult in the Senate, said one former senior Senate Democratic aide.

Reid at times has chafed over Pelosi’s approach, sometimes venting them in phone calls with House Majority Leader Steny Hoyer (D-Md.), a fellow trial lawyer by training whom Reid served with in the House for four years.

“Hoyer’s all about the horse-trade. He’s all about the deal and the compromise, and that’s what Reid’s about,” one Senate Democratic aide said. “Hoyer starts [conversations with Reid] by saying, ‘I know you have some compromises to make. … [Pelosi] starts conversations by saying, ‘You can’t change these things.’”

Reid also has been pained by Pelosi’s unwillingness to rein in House Appropriations Chairman David Obey (D-Wis.), who is viewed on the Senate side as sometimes erratic and pugnacious. That problem was made even more difficult by the fact that an ailing, 91-year-old Senate Appropriations Chairman Robert Byrd (D-W.Va.) was unable for much of the 110th Congress to act as an arbiter for the Senate’s point of view on critical appropriations legislation.

During negotiations on an omnibus spending bill in the fall of 2007, Reid was especially irritated by Obey’s decisions to publicly shift strategies without vetting it with Senate leaders.

The Senate Democratic aide said conflicts between chairmen present a special problem for Pelosi and Reid, and predicted that could come to a head when the House and Senate attempt to navigate a global warming bill to Obama’s desk.

“If there are philosophical differences between a chairman of the House and a chairman of the Senate, the role of the two leaders is to solve that problem,” the aide said. “In this case, sometimes the two leaders end up magnifying that difference.”

But the leaders’ relationship also includes plenty of respect for the other’s ability to thread the needle on delicate policy matters. Current and former Senate Democratic aides said Reid’s appreciation for Pelosi’s strength as a leader grew during the ultimately unsuccessful back and forth over ending the war in Iraq in the early days of the 110th Congress.

And Cardoza said “while I’m not saying she’s never been frustrated, I’ve never heard Nancy say a disparaging word about Mr. Reid.”

Their offices dismissed the notion that Reid and Pelosi have a sometimes-frosty relationship.

Reid spokesman Jim Manley said the Senator “understands the frustrations that House Members have about the Senate. This can be an unwieldy body … Sen. Reid shares that frustration.” Likewise, Pelosi spokesman Nadeam Elshami said the Speaker “understood clearly the challenges Sen. Reid faced every day with Republicans stalling or killing legislation that even had bipartisan support here in the House.”

Still, Manley agreed with Democratic Senators that House leaders should not expect wholesale changes in Reid’s ability to push legislation through the chamber in the new Congress.

“Fifty-nine is a heck of a lot better than 51, but the Senate rules are what they are,” Manley said. “This is a body where any one Senator can go to great lengths to gum up the works if he or she wants to.”

And Elshami expressed the House view that “things have changed.”

“We share the same agenda, and there’s a true sense of urgency, but also of working together in a bipartisan way to get things done.”