Senate Majority Leader Harry M. Reid (D-Nev.) and Senate Minority Leader Mitch McConnell (R-Ky.). (J. Scott Applewhite/AP)

Attempts to advance two spending measures to help fund federal agencies stalled this week in the U.S. Senate, possibly upending carefully orchestrated plans to approve a new spending agreement and avoid another government shutdown at the end of September.

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 bills that set funding for all corners of the federal government. Congress will need 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 this 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 an ongoing dispute with Republicans over plans to offer amendments to the bill.

Senate Majority Leader Harry M. Reid (D-Nev.) planned to require any amendment to pass with a super majority of 60 votes, saying that such a plan "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 to 30 percent in the coming years. But coal is a major source of energy and jobs in McConnell's Kentucky and in several other states represented by Democratic senators who are seeking reelection this year.

Aides said McConnell would seek any way possible to introduce his amendment and he planned to try to attach it this 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 Mikulski (D-Md.), who chairs the Appropriations Committee, and her House counterpart, Rep. Hal Rogers (R-Ky.),  had brokered a similar spending plan earlier this year and had been working in tandem to move bills through their respective chambers ahead of negotiations.

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 to noted that the appropriations panel has already 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.