The political stalemate that partially shut down the US government has worsened with Democrats rejecting the latest Republican effort to negotiate over efforts to derail the health care overhaul.
In a 54-46 party-line vote, the Democrat-controlled Senate turned aside the Republican House of Representatives' request to name negotiators to a conference to resolve differences. Senate Majority Leader Harry Reid said he would not negotiate as long as Republicans were using the tactic of holding up the straightforward spending bill which would keep the government operating.
About 800,000 federal workers are being forced out of work in the first government shutdown in 17 years, suspending most non-essential federal programs and services.
It was not clear how long the standoff would last, but there were no signs of compromise. The Senate vote marked the fourth time during this fight that it has rejected House Republican efforts.
Stock markets around the world reacted resiliently, with analysts saying significant damage to the US economy was unlikely unless the shutdown lasted more than a few days.
The stand-off pits Democrats and some mainstream Republicans against a core of conservative activists who have mounted a campaign to seize the must-do budget measure in an effort to derail the 2010 health care reform they have dubbed "Obamacare."
There are few issues Republicans feel as passionately about as the health care law. They see the plan, intended to provide coverage for the millions of Americans now uninsured, as wasteful and restricting freedom by requiring most Americans to have health insurance.
Until now, such temporary spending bills have been routinely passed with bipartisan support, ever since a pair of unpopular shutdowns 17 years ago engineered by then-Speaker Newt Gingrich severely damaged Republican election prospects and revived then-president Bill Clinton's political standing.
Mr Obama accused Republicans of holding the budget hostage to get what they want. "You don't get to extract a ransom for doing your job, for doing what you're supposed to be doing anyway, or just because there's a law there that you don't like," he said.
Republican leaders have voiced reservations about the effort and many predicted it would not work, fearing the public will blame them for the shutdown. Some even labelled it "stupid."
But individual Republican House members may face a greater risk by embracing a compromise. Many represent heavily partisan congressional districts, and voters in Republican primaries have ousted those they see as too moderate.
It was not clear how long the standoff would last, but it appeared that Mr Obama and Mr Reid had the upper hand.
"We can't win," said Republican senator John McCain, adding that "sooner or later" the House would have to agree to Democrats' demands for a simple, straightforward funding bill reopening the government.
Republicans are likely to take up the health care fight again when Congress must pass a measure to increase the borrowing cap, which is expected to hit its ceiling in mid-October.