WASHINGTON (AP) — Apparently fearing that the top Senate Republican might score a political win, Democrats for the second time in a week cancelled a preliminary vote on a major spending bill.

At issue was an amendment by Senate GOP leader Mitch McConnell that would have allowed the Kentucky Republican and Appropriations panel member to successfully go to bat for his state's coal industry as the spending panel was to consider on Thursday a measure funding the Energy Department and other agencies.

Senate Appropriations Committee Chairwoman Barbara Mikulski, D-Md., officially made the call Wednesday night as she separately struggled to open debate on a huge spending bill to fund five Cabinet departments for the upcoming budget year. It will require unanimous agreement among all 100 senators to open debate along the process Mikulski wants and such approval was lacking.

Combined, the twin developments demonstrated the difficulty Congress was having in managing the process governing the year-to-year decisions regarding agency budgets. Mikulski's measure would give senators a rare opportunity for open debate on legislation of any kind.

More interesting was the decision to block a panel vote on McConnell's pro-coal amendment. The Appropriations panel is stocked with Democratic red state supporters of the energy industry like Sens. Mary Landrieu of Louisiana and Mark Begich of Alaska, both of whom face difficult elections this fall.

McConnell's amendment appeared on track to prevail. It is aimed at blocking any new government rules on carbon emissions from coal-fired power plants. He's made his opposition to what he calls the Obama administration's "war on coal" a centerpiece in his own re-election bid.

Mikulski's decision to call off the vote on the underlying energy and water appropriations measure came a week after she cancelled debate on a huge measure that would also have forced endangered Democrats to vote on politically poisonous amendments regarding the implementation of President Barack Obama's health care law.

The broader spending bill had easily advanced Tuesday evening on a 95-3 test vote, but a subsequent agreement for debate terms proved elusive. Democrats controlling the Senate have for years followed their GOP predecessors in restricting the ability of the rank and file to offer amendments to legislation, under longstanding Senate traditions.

The hybrid $180 billion measure by Mikulski would fund the departments of Agriculture, Commerce, Justice, Transportation, and Housing and Urban Development for the 2015 budget year beginning Oct. 1, but complicated Senate rules require unanimous agreement to protect the budgets of the largest agencies from procedural challenges.

Senate action would be the first opportunity in several years for the chamber to have extended debate and offer floor amendments to an annual appropriations bill.

Republicans are sure to try to force votes that would make politically endangered Democrats squirm. And if Democrats try to quash such opportunities, they could seem high-handed. But the debate also promises votes on substantive issues such as revising hours of operation rules for the trucking industry.

The measure comes as the annual appropriations season is entering overdrive and key players like Mikulski are trying to revive the powerful Appropriations panel, which has been disrespected by both House Speaker John Boehner, R-Ohio, and Senate Majority Leader Harry Reid, D-Nev., in recent years.

The House, meanwhile, took up a $570 billion Pentagon funding measure Wednesday, hours after the House Appropriations Committee approved a bipartisan measure funding the Energy Department, Army Corp of Engineers water projects and the Defense Department's nuclear weapons program.

But earlier House subcommittee debate Wednesday on a measure slashing the Internal Revenue Service's budget was far more partisan, as was last week's floor debate on the Agriculture Department's measure, which opened the door for school districts to opt out of healthier school lunch standards — a top priority of first lady Michelle Obama.

"The real result of the (IRS) cut will be to simply prevent the agency from collecting money from tax cheats, and to increase our deficit as a result," said Rep. Jose Serrano, D-N.Y. But Serrano praised a small increase, to $230 million, for a fund that helps community development financial institutions provide loans in traditionally underserved inner cities and rural areas.