Much of the vast machinery of the U.S. government ground to a halt Tuesday as Democrats and Republicans blamed each other for a partial shutdown that closed federal agencies, national parks and research facilities across the nation.

Ominously, there were suggestions from leaders in both parties that the shutdown, heading for its second day, could last for weeks and grow to encompass a possible default by the Treasury if Congress fails to raise the nation's debt ceiling.

Speaking at the White House, President Barack Obama accused Republicans of causing the first partial closure in 17 years as part of a non-stop "ideological crusade" to wipe out his signature health care law.

House Speaker John Boehner disagreed: "The president isn't telling the whole story,' he said in an opinion article posted on the USA Today website. "The fact is that Washington Democrats have slammed the door on reopening the government by refusing to engage in bipartisan talks."

About 800,000 employees — about a third of the federal workforce — are being forced off the job in the first government shutdown in 17 years, suspending most nonessential federal programs and services. People classified as essential employees — such as air traffic controllers, Border Patrol agents and most food inspectors — will continue to work.

Among the furloughed workers were some at the National Institute of Health's famed hospital of last resort, where officials said no new patients would be admitted for the duration of the shutdown. Dr. Francis Collins, agency director, estimated that each week the shutdown lasts will force the facility to turn away about 200 patients, 30 of them children, who want to enroll in studies of experimental treatments. Patients already at the hospital are permitted to stay.

The shutdown began when Congress missed a midnight deadline Monday to pass temporary funding bill, stalled by conservative efforts to push through a delay in Obama's health law.

Senate Majority Leader Harry Reid said he would not negotiate as long as Republicans were holding up a straightforward spending bill to keep the government operating.

The Senate vote marked the fourth time during this fight that it has rejected House Republican proposals, including an initial attempt to defund the health care program altogether.

House Republicans answered the latest Senate vote with a proposal for legislation to reopen portions of the government, including iconic national parks like Yosemite and Yellowstone, while still demanding concessions on health care. The bills covered the Department of Veterans Affairs, the Park Service and a portion of the Washington, D.C., government funded with local tax revenue.

Democrats generally opposed all three, saying Republicans shouldn't be permitted to choose which agencies remain open and which stay shut. As a result, all fell well short of the two-thirds majority needed for expedited passage under House rules.

The White House also issued veto threats against the bills. White House spokesman Jay Carney said it was "not a serious approach."

Republican aides said all three bills that were sidetracked could be brought up again on Wednesday under rules requiring a mere majority to pass. They said the House might also vote on a measure to reopen the hospital at the NIH, after several Democrats cited the impact on patients.

White House communications director Jennifer Palmieri told MSNBC that the administration is open to changes in the health care law in future negotiations, but not as part of passing a budget bill.

Stock markets around the world reacted resiliently, with analysts saying significant damage to the U.S. economy was unlikely unless the shutdown lasted more than a few days. U.S. stocks edged higher Tuesday, while European stocks mostly recovered after falling the day before the shutdown deadline. Asian stocks were mixed.

The stalemate pits Democrats against a core of conservative small-government activists who have mounted a campaign to seize the must-do budget measure in an effort to dismantle the 2010 health care reform, which is intended to provide coverage for the millions of Americans now uninsured.

In the House, conservative Rep. Marsha Blackburn predicted the standoff would drag for several days. "People are going to realize they can live with a lot less government," she told Fox News.

Republicans passionately oppose the plan they have dubbed "Obamacare" as wasteful and restricting freedom by requiring most Americans to have health insurance.

Ironically, a major expansion of the health care law — the very event Republicans had hoped to prevent — was unaffected as consumers flocked for the first time Tuesday to websites to shop for coverage sold by private companies — with government subsidies available to many to lower their premium costs.

National parks, museums in Washington and agencies like NASA and the Environmental Protection Agency nearly closed. The National Zoo in Washington was closed to visitors and employees turned off the popular panda cam that had showed off a weeks-old cub.

Pentagon and administration lawyers were looking for ways to expand the number of Defense Department civilians who are exempt from furloughs, amid worries that that the shutdown is damaging U.S. credibility among its international allies, Defense Secretary Chuck Hagel said.

Until now, such temporary spending bills have been routinely passed with bipartisan support, ever since a pair of unpopular shutdowns in the winter of 1995-1996 severely damaged Republican election prospects and revived then-President Bill Clinton's political standing.

Republican leaders have voiced reservations about the effort and many lawmakers predicted it wouldn't work, fearing the public will blame their party for the shutdown. 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 lawmakers they see as too moderate.

Looming ahead is a potentially more dangerous fight: Republicans are likely to take up the health care fight again when Congress must pass a measure to increase the nation's borrowing cap. The U.S. risks a market-rattling, first-ever default on its obligations if Congress fails to raise that limit.

The White House was operating with a skeletal staff Tuesday, and officials were discussing whether Obama should change plans for a trip to Asia scheduled to begin Saturday.

The State Department continued processing passports and visas because that service is financed through fees. Embassies and consulates overseas were also open.

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Associated Press writers Andrew Taylor, Connie Cass, Donna Cassata, Alan Fram, Nedra Pickler and Seth Borenstein contributed to this report.