US Representative John Shimkus (R-IL) (2nd L) laughs as he talks to Representative Howard Coble (R-NC) (L) while arriving for a late night closed-door meeting of the House Republican caucus during a rare Saturday session at the US Capitol in Washington, September 28, 2013. [Photo/Agencies]

WASHINGTON - In a decision that could make a US government shutdown hard to avoid on Tuesday, the House of Representatives on Saturday prepared to reject an emergency spending bill approved by the Senate and push instead to delay President Barack Obama's healthcare reform law.

The vote by the Republican-controlled House was set to begin Saturday night. But there was little doubt about the outcome among Republicans, who cheered and chanted jubilantly in a meeting earlier Saturday after choosing their course of action.

Democrats were grim. "The government is going to shut down," said Representative Jim Moran of Virginia. "The only question now is for how long."

Senate Majority Leader Harry Reid, also a Democrat, said the Senate, which is not scheduled to meet until 2 pm EDT on Monday (1800 GMT), would not accept any funding measure aimed at derailing the 2010 healthcare restructuring known as Obamacare.

And Obama threatened on Saturday to veto any bill that contained such a measure.

The high-stakes maneuvering between Democrats and Republicans is likely to continue through much of Monday, with little time remaining before government funding runs out.

The impasse is the culmination of more than three years of failed conservative efforts to repeal "Obamacare," a health insurance program aimed at extending coverage to millions of those without coverage.

Republicans argue that "Obamacare, which is set to launch on October 1, is a massive and unnecessary government intrusion into medicine that will cause premiums to skyrocket and damage the economy.

They have attached a provision to delay the program to a "must-pass" bill that would continue funding the government when the fiscal year ends at midnight on Monday.

Failure to pass the bill would close down much of the government for the first time since 1996. More than a million federal employees would be furloughed from their jobs, with the impact depending on how long a shutdown lasted.

The current timetable could leave House Speaker John Boehner with the most difficult decision of his career: whether to approve a straight-forward spending bill passed on Friday by the Senate or allow the government shutdown to begin.

A shutdown could be averted, however, if 17 of the 233 House Republicans break from their party and vote with the Democrats.