Senate Democrats are laboring to produce a fiscal 2012 budget blueprint that will satisfy their own members and still draw a sharp contrast with a House Republican plan they have branded as extreme.

So far, achieving that goal has proved challenging.

Democratic leaders have to guard against moving too far to the left, because they want their budget to serve as a plausible starting point in negotiations over raising the debt limit.

A further complication: The blueprint needs to strike a balance between spending cuts and tax increases to unite liberal and moderate Democrats in a closely divided Senate.

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Both parties are working to produce a plan that would trim the deficit — thus making a vote to raise the $14.3 trillion debt limit more palatable — while not alienating their membership. While the Senate budget resolution might not be the final vehicle for any grand bargain on the debt limit, it does provide a window into the parties’ political considerations.

Democrats have slammed the House plan written by Budget Chairman Paul D. Ryan, R-Wis., but have left themselves open to charges they have no alternative to address the growing cost of entitlement programs and a record federal debt.

Early trial balloons from Senate Democrats suggest they are looking at substantial tax increases as a way to curtail the debt, a move that GOP leaders in both chambers categorically have ruled out. Reconciling those two positions will be crucial for Congress to strike a debt limit deal before early August, when Treasury Department officials have warned the nation could face a default on its borrowing obligations if the limit is not raised.

The House adopted its fiscal 2012 budget resolution (H Con Res 34) on April 15. But in the Senate, Budget Chairman Kent Conrad, D-N.D., has yet to present a fiscal 2012 blueprint to his committee. The House plan would cut spending by $5.8 trillion over 10 years and reduce the deficit by $1.6 trillion over that period.

Getting a plan through the Senate Budget Committee, much less the full chamber, is proving to be an undertaking.

Conrad was forced this week to rewrite a $4 trillion deficit reduction plan he had earlier presented to Democratic senators after Vermont independent Bernard Sanders threatened to block its approval in committee.

In order to move a plan out of the committee, Conrad needs the vote of every Democratic member as well as Sanders, who caucuses with Democrats, assuming all Republicans on the committee oppose it.

Responding to Sanders’ complaints that the earlier proposal was too easy on the wealthy, Conrad tilted his latest plan to include more tax increases and fewer spending cuts, according to a Senate aide who has seen details of the plan.

The blueprint — which Conrad presented to Democratic colleagues May 10 — would call for raising taxes by about $2 trillion over a decade while cutting spending about $1.5 trillion, according to the aide.

By emphasizing revenue increases over spending cuts, Conrad may be angling to win support from Sanders and other liberals. But if he goes too far, he risks alienating Democratic moderates wary of large tax increases.

With only 53 Democratic Caucus members, Majority Leader Harry Reid, D-Nev., can lose only three votes if all Republicans were to vote against the Democratic budget resolution, as is customary. Vice President Joseph R. Biden Jr. would then have to cast the tie-breaking vote for adoption.

“I would hope there would be an effort at deficit reduction based on reduction of spending,” said moderate Democrat Ben Nelson of Nebraska on Wednesday. “Tax code reform may be appropriate, but I’m focused on what the cuts will consist of.”

Sanders pressed his case May 10. “What we’re dealing with is a right-wing extremist budget coming out of the House,” he said. “We need a counterbudget which makes sense to ordinary Americans.”

Asked if Democrats could reach agreement on such a budget, Sanders said, “We’re working on it.”

Conrad declined to confirm or deny details of the budget plan, saying that he has “shared various draft budget proposals with colleagues” and the “draft budget has changed and will continue to evolve as I incorporate feedback from senators.”

Conrad’s deficit-fighting plan is expected to be included in a fiscal 2012 budget resolution that he hopes to release for a markup May 18.

His latest draft calls for cuts of $900 billion from defense and $300 billion from non-security discretionary spending over 10 years. It envisions cuts of $300 billion from mandatory spending programs. While specifics have yet to be released, Conrad has said Social Security would not be affected.

The plan relies on about $600 billion in savings from lower interest payments on the debt to achieve $4 trillion in deficit reduction. If the interest savings are viewed as spending cuts, total savings from spending cuts is about $2 trillion.

Conrad also plans to include a 3 percent surtax on millionaires, although specifics of that proposal remained unclear.

Earlier this year, Sanders introduced legislation (S 552) to clamp a 5.4 percent tax on adjusted gross income exceeding $1 million. He said the tax would raise as much as $50 billion a year, or roughly $500 billion over 10 years.

Republican leaders in both chambers have vowed to oppose a debt limit increase unless it is tied to debt reduction measures and spending cuts. They have also promised to oppose any deal that raises taxes.

Democrats have started to tack on some of their own conditions, with Charles E. Schumer of New York, the third-ranking Democrat in the Senate, saying Wednesday that Democrats will insist that revenue-raising measures, such as a repeal of oil and gas industry tax breaks, be tied to a debt limit increase.

The White House has played an increasingly visible role in the deficit talks.

After meeting with Senate Democrats on Wednesday, President Obama plans to meet with Senate Republicans on Thursday as part of his campaign to win GOP support for raising the debt ceiling.

Schumer credited the administration for outreach. “I think the president’s budget really hit the sweet spot for the caucus — a mixture of cuts and some revenue increases based on no tax cuts for the rich,” he said.

Meanwhile on Wednesday, some of the biggest names in the business and financial world officially jumped into the debate over the country’s fiscal path with a letter to congressional leaders warning that “failure to increase the statutory debt limit in a timely fashion could have a significant and long-lasting negative impact on the U.S. economy.”

The letter, which was expected for some time, was signed by several dozen groups, including the U.S. Chamber of Commerce, National Association of Manufacturers, Financial Services Roundtable and Securities Industry and Financial Markets Association.

The groups said “tough calls on U.S. spending must be made as part of a debate about the budget, and we agree that restoring balance to our fiscal position will require that the government spend less and spend more wisely.”

Brian Friel, Jennifer Scholtes, Ben Weyl and Joseph J. Schatz contributed to this story.