The first thing to know about the Keystone pipeline? It already exists. Here's a breakdown of the pipeline's various parts. (Gillian Brockell, Jhaan Elker and Kate M. Tobey/The Washington Post)

The impassioned debate over the Keystone XL pipeline could reach a tipping point this week on Capitol Hill, with this likely the last chance for the issue to be considered legislatively until after the November midterm elections. The outcome could complicate matters for the Obama administration, which is still reviewing whether to allow permits for sections of the pipeline.

The Senate is set to begin debate on a modest energy bill that has enough bipartisan support to pass on its own merits, but supporters of the pipeline, which is intended to transport oil from western Canada to the U.S. Gulf Coast, are trying to leverage the bill to force votes on Keystone. The issue has prompted a multimillion-dollar lobbying campaign on both sides; hardly a half-hour goes by on cable news outlets without one side or the other pushing its position on the matter.

A handful of Democrats facing tough reelections are supportive of the new pipeline. That has led Senate Majority Leader Harry M. Reid (D-Nev.) to engage in sensitive negotiations over the parliamentary process about how amendments to the bill — drafted by Sens. Jeanne Shaheen (D-N.H.) and Rob Portman (R-Ohio) — will be considered.

GOP leaders have pushed to get an amendment on the Shaheen-Portman bill that would allow them to simply mandate the construction of the pipeline, overriding the ongoing review by the State Department as it considers how to handle 2.5 million public comments on the proposal and an ongoing lawsuit in Nebraska trying to alter the route. That review, with a further delay announced two weeks ago, seems certain to last past November and possibly into next year.

Reid offered Republicans a separate vote on a bill that would be a clean Keystone vote, nothing else, requiring 60 votes to overcome the filibuster of liberal senators opposed to the proposal. “The key then is getting 60 votes,” Sen. John Hoeven (R-N.D.), the co-sponsor with Sen. Mary Landrieu (D-La.) of the Keystone amendment, told reporters Thursday.

Landrieu is chairman of the Senate Energy and Natural Resources Committee and one of the most endangered incumbents seeking reelection this fall.

By the end of the week, Hoeven said he had not secured at least 60 votes, to some degree because of the confusion surrounding whether it would be part of the Shaheen-Portman bill or a freestanding vote. Some Democrats who have previously supported the bill have been hesitant to support a Keystone amendment, out of fear that Obama would veto the entire package because of the controversial measure, torpedoing the underlying energy legislation.

The most recent Senate vote in support of the proposed pipeline produced a filibuster-proof majority of 62 votes, but it was a nonbinding vote during the 2013 consideration of the chamber’s budget resolution. All 45 Republicans as well as 17 members of the Democratic caucus approved the sense-of-the-Senate resolution at the time.

If that same coalition held together and approved the proposal, Hoeven said, House Republicans would be expected to quickly approve the Keystone XL project and send it to Obama, daring him to veto a plan supported by almost a third of Senate Democrats and a clutch of incumbents critical to holding the majority in November.

However, such a vote is not certain. Reid’s offer is contingent upon an agreement on other amendments that Republicans want to offer on the Shaheen-Portman bill, and that’s where the pipeline’s political mythology runs up against the reality of its significance in the energy universe.

At its core, the debate is about producing energy, and jobs, versus environmental protections. Yet Keystone’s actual impact is far less significant in those areas and instead has become much more politically symbolic for the sides engaged in this fight.

Even the rosiest estimates predict just 9,000 jobs would be created by allowing this nearly 1,200-mile pipeline to be constructed from western Canada down to Nebraska — a nice bump but not exactly a game-changer for a domestic economy that created 288,000 jobs in April.

Environmental opponents decry that the ensuing 830,000 barrels of oil coursing through the proposed pipeline each day would be carrying a dirty batch of tar sands oil — an alarming figure, yet the first three phases of Keystone pipelines have already been approved and have the capacity to deliver 1.3 million barrels a day through a longer route to the Gulf Coast.

Far more critical to environmental activists are rules governing carbon emissions, an ongoing battle between the Environmental Protection Agency and the power industry that still uses coal-burning plants in the Midwest and other regions.

Hoeven said that one of the amendments Republicans would like to advance is a bill that House Republicans just passed, largely on party lines, reining in EPA powers in regulating carbon in some plants.

On Thursday, senior Democrats signaled that votes such as that could be deal-killers to the overall plan to allow amendments on Shaheen-Portman, and if that deal fell through then the proposal to allow a Keystone vote would also collapse.