These are some legitimate concerns, and it will be difficult to hammer out a consensus on some of them - especially the proposal to "means test" Medicare drug benefits. This will be a real rest of Reid's and Pelosi's leadership:

Democratic leaders in Congress did not expect much Republican support as they pressed President Obama's ambitious legislative agenda. But the pushback they are receiving from some of their own has come as an unwelcome surprise.

As the Senate inches closer to approving a $410 billion spending bill, the internal revolt has served as a warning to party leaders pursuing Obama's far-reaching plans for health-care, energy and education reform.

Those goals, spelled out in Obama's 2010 budget blueprint, continue to enjoy broad Democratic support. But as the ideas develop into detailed legislation, they will transform from abstract objectives into a tangle of difficult trade-offs. Crop subsidies, the student loan program and Medicare radiology rules are all currently niche concerns, but any one could become the next crisis for party leaders, with the potential to derail a major agenda item. One major proposal, to limit itemized deductions for wealthy taxpayers, has already raised doubts among prominent Democrats in both chambers.

Some issues that inflamed Democrats in previous years have yet to even register, including the proposal in Obama's budget plan to "means-test" the Medicare drug benefit as a way to pay for health-care reform. Doling out entitlement benefits based on income has long been anathema to most Democrats.

"There are a lot of items in the budget that would normally get a lot more attention, if we were in a normal year," said Rep. Chris Van Hollen (D-Md.), who heads the House Democrats' fundraising arm. "They've been eclipsed by the tidal wave of the economy." But Van Hollen added: "They are waiting in the wings."

• Sen. Bob Menendez (D-NJ) was furious over a provision that would allow travel and imports to Cuba, saying that making such a major policy change in a spending bill was "undemocratic."

• Sen. Ben Nelson (D-NB) is upset about a provision that overhauls the student loan program and ends federal support for private lending. One of the major lenders is a company based in Nebraska, and Nelson says it will cost his state 1,000 jobs.

• Nelson also objects to changes Obama has proposed in the farm subsidy system that would stop direct payments to farms with annual sales of more than $500,000. Senate Budget Committee Chairman Kent Conrad (N.D.) also opposes it.