Democrats and Republicans once again fighting about the spending bills, stifling hopes for quick progress. (JOSHUA ROBERTS/REUTERS)

A fresh fight between Democrats and Republicans over rules regarding amendments broke out in the Senate last week, possibly upending carefully orchestrated plans to approve a new spending agreement and avoid another government shutdown this fall.

After several years of rancorous debate over the nation’s fiscal policy, House Republicans and Senate Democrats have been working at a notably productive pace this year to pass the appropriations bills that set funding for all corners of the federal government. Congress will need to have a new spending plan in place — either a short-term or year-long agreement — before the next fiscal year begins Oct. 1. The House began debating the spending bills in May — the earliest it has done so since 1974 — and the Senate started this month with plans to hold up to four weeks of debate on the bills this summer, a considerable amount of floor time for any topic in the modern age.

But Senate Democrats last week postponed plans to move ahead with a $180 billion proposal to fund the departments of Agriculture, Commerce, Justice, Transportation, and Housing and Urban Development amid a fresh dispute with Republicans seeking to amend the bill.

Senate Majority Leader Harry M. Reid (D-Nev.) planned to require amendments to pass with a supermajority of 60 votes, saying that the approach “will not come as a surprise to anyone in this chamber” because Republicans have insisted on such thresholds to advance other legislation in the past.

But Senate Minority Leader Mitch McConnell (R-Ky.) objected, saying that the rules would make it impossible for Republicans to seek changes to the spending bills.

McConnell is running in one of the most closely watched reelection contests this year and has been seeking to block the Obama administration from implementing new rules to curb carbon emissions from coal-fired power plants. President Obama plans to cut carbon dioxide emissions from existing coal plants by up to 30 percent in the coming years. But coal is a major source of energy and jobs in McConnell’s state and in several others represented by Democratic senators who are seeking reelection this year.

Aides said McConnell would seek any way possible to introduce his amendment to block the new rules for coal plants and that he planned to try to attach it last week to a spending measure funding the Energy Department and related agencies. But Democrats, realizing that McConnell’s amendment likely would pass with the votes of some Democratic senators despite Obama’s objections, postponed a hearing on the bill.

“Once again, Senate Democrats are preventing my commonsense, pro-coal measure from moving forward,” McConnell said in response. “They’re doing the bidding of the administration, instead of listening to constituents back home.”

The dispute is just the latest in a long-running battle between Democrats and Republicans over control of the fractured Senate. Members of both parties had hoped that the appropriations process would be exempt from the floor fights, in part because Sen. Barbara A. Mikulski (D-Md.), who chairs the Appropriations Committee, and her House counterpart, Rep. Harold Rogers (R-Ky.), brokered a similar spending plan this year and have been working to move bills through their respective chambers at a brisk pace.

Mikulski on Thursday described the impasse as “sad” but said that she remains hopeful that Reid and McConnell can work out an agreement. Her aides noted that the appropriations panel has approved six spending bills with bipartisan majority votes. Vincent Morris, a spokesman for the committee, called the impasse “a little bump in the road, and we intend to roll up our sleeves and come back at this again next week.”

But next week is the first of just five weeks left before the five-week summer recess — and there are only 10 legislative days scheduled in September before funding dries up.