WASHINGTON — The Senate began debate Tuesday on a bill that supporters of the Keystone XL oil pipeline want to use as a vehicle to authorize immediate construction of the proposed pipeline from Canada to the United States.

The 79-20 vote to limit debate on an energy efficiency bill lost much of its significance as the two parties continued to argue over what amendments to allow on the measure, including the pipeline project.

Senate Majority Leader Harry Reid, D-Nev., denied a Republican request for an amendment on the pipeline, but said he is open to a stand-alone vote on a pipeline bill later.

Reid accused Republicans of trying to block the energy bill, which has bipartisan support. Republicans said Reid was backing away from a promise to allow a vote on Keystone.

“Senate Republicans keep changing their requests,” Reid said, noting that some Republicans first asked for a “sense of the Senate” resolution on Keystone and then later called for a binding vote.

“It seems like this is nothing but a game of diversion and obstruction to many Senate Republicans,” Reid said on the Senate floor. “But it’s not a game. Every time a group of Republicans feigns interest in bipartisanship, only to scramble away at the last moment, it is part of a calculated political scheme.”

Senate Minority Leader Mitch McConnell, R-Ky., called Reid’s claim “laughable,” and said all that Senate Republicans seek is a full and open debate on energy policy.

“The American people have waited seven long years for a serious energy debate in the Democrat-run Senate,” McConnell said, noting that the Senate has not approved a major energy bill since 2007.

In addition to Keystone, Republican senators have prepared a host of amendments to the energy bill, including one that would block the Environmental Protection Agency from imposing rules limiting greenhouse gas emissions from coal-fired power plants. Lawmakers from both parties also support a measure to speed approval of terminals to export liquefied natural gas.

“The American people deserve a real debate on how we can best tap our own extraordinary natural resources to achieve energy independence at home and how we can help our allies overseas through increased exports of American energy.

But we can’t move forward if the Democrats who run the Senate keep trying to protect the president at the expense of serving their constituents,” McConnell said.

Sen. Jeanne Shaheen, D-N.H., one of the bill’s co-sponsors, said the measure was an affordable approach to boost energy efficiency, which she said is the best way to save money on energy use.

“Energy efficiency is no longer about putting on a sweater and lowering the thermostat. It’s about the technologies that can reduce energy use,” she said.

Sen. Rob Portman, R-Ohio, said the bill does not include mandates but merely encourages homes and businesses to increase efficiency.

“The least expensive form of energy is the energy we don’t end up having to use,” he said.