Republicans in the Senate are hoping to move their tax plan before the end of the year, but with such a narrow majority, party leaders cannot, for now, afford more than two defections. Wisconsin's Ron Johnson has already announced his opposition to the current version of the bill. If a couple more of his colleagues follow suit, the ship, as currently constructed, sinks -- and with it, the Trump administration's last hope of scoring a landmark legislative win in its first year.

Flake has expressed reservations of his own about the broad strokes of the bill, which would raise deficits over time. He's yet to give any clear indication on how he intends to vote, but that didn't stop President Donald Trump on Sunday from firing off a pre-emptive shot.

"Sen. Jeff Flake(y), who is unelectable in the Great State of Arizona (quit race, anemic polls) was caught (purposely) on 'mike' saying bad things about your favorite President," Trump tweeted . "He'll be a NO on tax cuts because his political career anyway is 'toast.'"

Flake had, on Friday, been caught by a live microphone after a town hall in Mesa, Arizona , saying that, if Republicans "become the party of Roy Moore and Donald Trump, we are toast." Left unsaid here is the reality that, by passing a tax plan Trump has consistently hyped, Republicans would move themselves further in that direction.

Twitter drama aside, the decision facing Flake on the tax bill represents his first real opportunity to deal the administration, whose behavior he described last month as "dangerous to a democracy," a staggering political kidney shot.

Doing so would of course require blocking the massive corporate tax cuts that so many Republicans, not just Trump loyalists, are desperate to pass ahead of the midterms. Failure to deliver there, some in the GOP quite openly fear, would further anger already frustrated and impatient supporters -- and risk drying up campaign coffers ahead of the coming elections.

A battering in 2018 would effectively halt the broader Republican agenda. If Democrats regain control of the House, there is little Trump and his Senate allies (a tenuous coalition in any event) could do to move legislation without engaging in serious bipartisan negotiations.

For his entire career in politics, Flake has graded out as one of the Republican Party's most conservative members. Leaving tax cuts of the magnitude currently drawn up by his colleagues on the table would surely gnaw at him. The prospect of it clearly does. But there is -- according to the senator himself -- something larger at stake here.

If the tax plan fails, the administration could be irreparably compromised. Top officials like Gary Cohn, Trump's top economic adviser, might go looking for an escape hatch. Republicans on Capitol Hill might feel compelled -- and emboldened, in some cases -- to more forthrightly examine the President's behavior, before and after the election. The warning Flake sounded in late October would, with a no vote, come closer to assuming the implied weight of its words.

Such a decision would not even require Flake, a noted deficit hawk, to abandon his own stated political principles. In fact, it would have the effect of affirming them. The only thing undermined, or diminished, would be Trump's sway with Republicans in Washington.

As Flake put it after the Senate proposal was unveiled: "I remain concerned over how the current tax reform proposals will grow the already staggering national debt by opting for short-term fixes, while ignoring long-term problems for taxpayers and the economy."

Passage of the plan would not guarantee Trump a surge in the polls, or heal the wounds of the last 10 months, but it would undeniably buy the White House time and goodwill with conservatives. A move to block it would be a consequential rebuke that could bring the GOP closer to the reckoning he and other Trump-skeptic officials claim to desire -- and stand as a worthy rejoinder to critics who rolled their eyes at Flake's October 24 remarks.

"The principles that underlie our politics, the values of our founding, are too vital to our identity and to our survival to allow them to be compromised by the requirements of politics," Flake said then. "Because politics can make us silent when we should speak, and silence can equal complicity. I have children and grandchildren to answer to, and so, Mr. President, I will not be complicit."

Here's his chance.