At the marathon Senate Budget Committee hearing this week, Sen. Bob Corker, R-Tenn., strolled in like a man who had just quit his job and was ready to tell the boss what he really thinks.

"I have difficulty putting that much energy into discussing this budget document because this is some of the most meaningless work that we do here," Corker said with a sigh. "It has nothing to do with the chairman or the committee. It's a waste of time."

Corker announced last week that he plans to retire at the end of this Congress. He has never been slow to voice his opinions — even ones that go against the party line — but he seems to be speaking even more freely since he announced he won't run for re-election.

"I mean, I still want to be myself here. I don't want to all of sudden, I'm leaving and I act differently," Corker told reporters when asked about the apparent uptick in his candor on Capitol Hill. " 'Do you feel a little bit more [free]?'I don't know, maybe."

Corker drew a lot of attention this week with his critical comments about the "chaos" in the Trump administration on foreign policy and tensions inside the State Department.

But Republicans concerned about the party's scorecard for legislative victories this year should be more focused on Corker's fresh warning signals that he is not fully on board for the GOP's early outline of tax legislation.

At committee hearings, to party leaders, at private Senate lunch meetings and to the media this week, Corker has laid bare his concerns about a tax bill that threatens to add to the federal budget deficit.

"Unless it reduces deficits — let me say that one more time — unless it reduces deficits and does not add to deficits with reasonable and responsible growth models, and unless we can make it permanent, I don't have any interest in it," Corker said at the Budget Committee hearing.

Republicans have outlined the framework for their tax bill that aims to reduce the corporate and individual tax rate, collapse individual tax brackets from seven to three and eliminate certain deductions to help pay for it.

The questions that Corker raises — whether it will add to the deficit, how much economic growth it will produce and whether the changes will expire or be made permanent — have not yet been answered.

Republicans first need to pass a budget resolution, the annual political exercise Corker derided as a "waste of time" at this week's hearing because of the hours of theatrics required to pass a resolution along a predictable party-line vote.

The resolution, once approved by both chambers, will pave the way for Senate Republicans to pass tax legislation that only requires 50 votes. It is that same process Republicans used to try to pass health care legislation without needing any Democrats.

Just like on health care, Republicans can only lose two votes and still pass a tax bill if no Democrats support it. That is why securing Corker's vote is critical.

Senate Finance Committee Ranking Member Ron Wyden, D-Ore., who also sits on the Budget Committee, was there for Corker's remarks this week. He says Republicans risk defeat because they are employing the same tactics they used on health care to try to get a tax bill passed.

"Let's put it this way: In the health care bill, a number of senators just point blank said this kind of [bill] — where you're just to drive it through on a partisan basis, where you don't have any [Congressional Budget Office] score, where you have really fanciful notions with respect to the revenue and the like — a bunch of them said, 'Nothing doing,' and the bill went down," Wyden said, in reference to Sens. John McCain of Arizona and Susan Collins of Maine, who were decisive "no" votes on health care.

"Now usually you learn from something that turned out to be a debacle, but as of now, they seem inclined to go down the same path," Wyden said.

Senate Republicans believe that they will write a tax bill Corker can support.

"I think Sen. Corker is one of the brightest guys in the entire conference. I think he is sincere in his concerns. I'd much rather have him expressing that now than in the last few days before a vote," said Sen. Mike Rounds, R-S.D., "He adds value, and he will make our product better."

Most Republicans freely admit that the political pressure has only ratcheted up to pass a tax bill after the party's failure to repeal the Affordable Care Act. Corker is notably not one of them. "Some people may be feeling we have to do tax reform, even if we do the wrong kind of tax reform, just to deliver," he said, "This is about substance to me; it's not about politics."

The substance on the deficit is this: In the most recent four fiscal years, the federal government spent roughly between $400 billion and $600 billion more than it took in every year. In September, the U.S. national debt crossed a staggering $20 trillion mark for the first time.

"It's really disheartening to watch how many people are putting the debt second to their desire for big tax cuts when what we need to do is reform the tax code," said Maya MacGuineas, who runs the Committee for a Responsible Federal Budget, a fiscal watchdog group. "If we have a tax cut right now at a time when the economy doesn't need stimulus and our debt is at near record levels, that will do a lot of damage for the economy and it will be a huge missed opportunity."

For deficit hawks like Corker, it is unconscionable to vote for tax cuts if they believe it means more deficits and more debt. "This is the most passionate thing for me, period, that I work on. Not foreign policy, not banking. It's this deficit issue," said Corker, who is the chairman of the Senate Foreign Relations Committee. "And through the years, especially after Election Day, I've just seen it's like party time up here. Nobody cares about deficits anymore."

Republicans who loudly criticized the Obama administration for deficit spending are more muted now that the GOP is in control. One of the most prominent ones is Budget Director Mick Mulvaney, who was a vocal deficit hawk when he served in Congress but this week said "new deficits" could be necessary to achieve higher rates of economic growth.

"If we simply look at this as being deficit-neutral, you're never going to get the type of tax reform and tax reductions that you need to get to sustain 3 percent economic growth," he told Fox News Sunday.

That argument is unlikely to win over Republicans like Corker, who repeated this week that he views the deficit as the biggest threat to the nation.

"I'm doing what I can hopefully in a nice way to try and steer us back to the fact that deficits matter. They're a greater threat to us than North Korea or [the Islamic State]," he said.

And Corker could be a bigger threat to the Republican tax bill than the party realizes.

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