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Mr. Toomey said he is willing to look at streamlining the tax code to remove distorting tax breaks, such as those that go to ethanol promoters. But he rejected rate increases.

“If we tackle tax policy, and I hope we will be able to make progress on tax policy, I think the goals should be to broaden the base and lower rates,” he told reporters on a conference call.

Analysts said the selections so far suggest a committee that will struggle to find common ground, and with a few exceptions there are not many deal-makers or compromisers among the nine known members.

Three of the nine — Democrat Mr. Baucus and Republicans Mr. Camp and Mr. Hensarling — voted against the final December report from President Obama’s deficit commission, which recommended a mix of spending limits and cancellation of tax breaks.

Meanwhile, none of that commission’s “yes” votes will be part of this committee, nor will any of the members of the so-called “Gang of Six” senators, who independently crafted their own deficit-reduction package last month.

Meanwhile, Mrs. Murray’s selection drew outrage from Republicans, who said that because she also runs Senate Democrats’ campaign committee, it inevitably will inject politics into the conversation.

Sen. John Cornyn of Texas, who runs Senate Republicans’ campaign committee, said he told Mr. McConnell not to select him so he can keep the panel above reproach.

White House press secretary Jay Carney rejected the concerns as “silly criticism” and said Mr. McConnell has said his chief goal in politics is to defeat Mr. Obama.

“That’s OK because we know that we can put aside a lot of the partisan rhetoric and focus on what needs to get done,” Mr. Carney said.