Rep. Greg Walden at the 2016 Republican National Convention. Julie Sabatier/OPB

Rep. Greg Walden, Oregon's only Republican in Congress, is losing his spot on the conference committee that will iron out the differences between the Senate and House versions of the big tax-cut legislation.

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What's not clear is exactly why he exited the committee just a day after he was appointed to the panel Monday by House Speaker Paul Ryan.

The official announcement on Tuesday referred only to the "removal of the gentleman from Oregon" and the appointment of Rep. Fred Upton to the committee.

Aides to both Walden and Upton said in separate statements that Upton chairs an energy subcommittee that deals with one of the provisions under debate. This is language added to the Senate bill that authorizes sales from the Strategic Petroleum Reserve to help pay for the tax cut.

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That provision "falls directly under Rep. Fred Upton's role" as subcommittee chairman, said Walden spokesman Justin Discigil.

However, the subcommittee is part of the House Energy & Commerce Committee, which Walden chairs. And his committee will also oversee negotiations on a much weightier issue: the individual health care mandate.

The Senate bill calls for eliminating the Obamacare mandate requiring people to obtain health coverage or face fines. The Congressional Budget Office says this would save the government an estimated $338 billion over the next 10 years that could be used to help pay for the tax cuts.

But the Congressional Budget Office also estimates that there would be another 13 million people without health coverage in the next decade.

Some of the reduction would come because more people would not buy coverage without the mandate. But much of it is because it’s estimated to drive up insurance costs as healthier people drop coverage – thus making it harder for others to afford insurance.

Supporters of the provision say the mandate is unpopular and unfair. Critics say it’s another attempt to destabilize Obamacare. Walden played a key role in shepherding a bill through the House earlier this year aimed at repealing and replacing the Affordable Care Act.

But Discigil didn’t respond to questions about why Walden isn't on the conference committee so he can deal with the mandate.

Upton and Rep. John Shimkus, R-Illinois, who is also serving on the conference committee, are both members of the energy and commerce’s subcommittee on health, so they also have experience with the issue.