* Speaker must decide whether to allow a House vote
* Boehner's likely choice: default or alienate Tea Party
* Would he allow a bill with mostly Democratic support?
If the Democrat-led U.S. Senate manages to strike a deal to reopen the American government and avert a catastrophic default on its debt by Thursday's deadline, the agreement - and the U.S. economy - is likely to hinge on one man: John Boehner.
Boehner, the Republican speaker of the U.S. House of Representatives, could be about to confront the most critical decision of his three decades in politics.
He could allow the House to pass a Senate deal that likely would get more support from Democrats than Republicans - a move that almost certainly would lead some conservatives to push for Boehner's removal as speaker - or he could side with rebellious Tea Party Republicans by preventing a vote and allowing America to default.
"John Boehner is caught between a rock and a hard place," said Ford O'Connell, a Republican strategist.
It's a familiar place for Boehner, who often has struggled to manage several dozen conservative Tea Party Republicans who have shown little interest in compromising with the Senate or Democratic President Barack Obama.
Boehner's current plight is largely a result of his desire to placate the vocal Tea Party caucus and adhere to the "Hastert Rule," a tactic named after Republican Dennis Hastert, who was House speaker from 1999 to 2007.
It is not a formal rule in the House, but stems from Hastert's general policy of not allowing a bill to be voted on by the full House unless most members of the House's majority supported it. Hastert wanted to avoid a repeat of complaints about his predecessor, Republican Newt Gingrich, whose management style was seen by some Republicans as too dictatorial.
Hastert, now an adviser to a Washington law firm, told The Washington Post this month that the term "Hastert Rule" is a "misnomer," but said the important thing in pushing legislation as a Republican speaker is "to move your party's philosophy," rather than advance Democrats' agenda.
Hastert's successor as speaker, Democrat Nancy Pelosi, made a point of rejecting the Hastert Rule and allowed some bills to pass with more support from Republicans than Democrats, including one that funded the Iraq war.
Boehner, however, has seemed to embrace the Hastert Rule.
His following of it is a big reason why he has refused to allow Senate bills that would reopen the U.S. government and raise the government's $16.7 trillion borrowing limit to be considered by the House.
If he had allowed votes on such measures they likely would have passed with most support coming from Democrats, members in both parties say, and the current crisis could have been averted.
But that would have required Boehner to essentially nullify conservative House Republicans' strategy of holding up funding for government operations and delaying an increase in the government's legal debt ceiling to try to force Obama and the Democrats to dismantle or delay the president's signature healthcare overhaul.
Absent action by Congress, much of the U.S. government has been shut down for two weeks and the U.S. Treasury is warning that the government will start to run out of money to pay its bills on Thursday, unless Congress extends its borrowing authority.
'STRONG REPUBLICAN SUPPORT'
Michael Steel, a spokesman for Boehner, said that like every speaker, Boehner's goal "is to pass bills with strong Republican support."
As for a deal with the Senate, Steel said: "We'll have a look at it and make a decision."
Steel noted that Boehner had allowed major legislation to pass the House without majority Republican support during the past year, notably as part of the budget deal that helped the government avoid a "fiscal cliff" in January.
That move drew outrage from Tea Party Republicans, who threatened to oust Boehner in last January's vote for speaker - a threat that never really materialized.
But Tea Party supporters who see the current stalemate as their best chance to undermine Obamacare are making it clear that Boehner will pay a price for allowing such a vote again.
Jenny Beth Martin, co-founder of the Tea Party Patriots advocacy group, said that if Boehner allows any deal through the House that does not dismantle Obamacare, "he may need to be concerned over his own party calling for new leadership. The Tea Party has been in this because of Obamacare, and he needs to understand that."
BEGINNING OF THE END
For Boehner, the political calculus behind such a decision is clear.
In the House, there now are 232 Republicans and 200 Democrats, with three vacant seats. A bill needs 217 votes to pass.
Conservative Republican members who were endorsed by Tea Party groups account for about one-third of the Republican caucus. In all, 160 Republicans in the House get high marks from the ultra-conservative Club For Growth.
The support for conservatives in the House makes it unlikely that any Senate deal to end the shutdown or raise the debt ceiling would be supported by a majority of House Republicans because it would not include changes to "Obamacare" or spending cuts that Republicans want, said David Gergen, who has advised Republican and Democratic presidents.
Gergen said that Boehner "will have to decide whether to forget the Hastert Rule, and allow Democrats to vote" on any deal that emerges from the Senate.
"The question for Boehner is whether to fall on his sword and prevent a default, or take the country into default because of the Tea Party," Gergen said. "At the end of the day ... I think he'll fall on his sword, even it means the end of his speakership. If he does that, I do think his days as Speaker will be numbered."
Steve Bell, a Republican Party veteran and chief of staff on the Senate Budget Committee during Republican Ronald Reagan's presidency, said Monday: "I have known John Boehner for 20 years. He is not going to let this country default."
Chris Van Hollen, a Democratic congressman from Maryland who has negotiated with Boehner on budget matters since 2011, said: "The health of the American economy depends on his putting country over his own job. This is the time Speaker Boehner has to cut his umbilical cord with the Tea Party."