WASHINGTON — Hours before a threatened government shutdown, the Senate has the next move today on must-do budget legislation that has fueled a bitter congressional dispute over President Barack Obama’s signature health care law.

But the Senate won’t be riding to the rescue, at least not immediately. When it convenes at midday, the Democratic-led chamber was expected to reject the latest effort from House Republicans to use a normally routine measure to attack “ObamaCare.”

If no compromise can be reached by midnight, Americans would soon see the impact of a government shutdown. National parks would close. Many low-to-moderate income borrowers and first-time homebuyers seeking government-backed mortgages could face delays.

About 800,000 federal workers would be forced off the job without pay. Some critical services such as patrolling the borders, inspecting meat and controlling air traffic would continue. Social Security benefits would be sent, and the Medicare and Medicaid health care programs for the elderly and poor would continue to pay doctors and hospitals.

Senate rejection would send the measure back to the House, where conservative Republicans want to delay by a year key parts of the new health care law and repeal a tax on medical devices as the price for avoiding a shutdown. A House GOP leader, Rep. Kevin McCarthy of California, said the House would rebuff the Senate’s efforts to advance the short-term funding bill as a simple, “clean” measure shorn of anti-heath care reform provisions.

Since the last government shutdown 17 years ago, temporary funding bills known as continuing resolutions have been noncontroversial, with neither party willing to chance a shutdown to achieve legislative goals it couldn’t otherwise win. But with health insurance exchanges set to open Tuesday, tea party Republicans are willing to take the risk in their drive to kill the law.

“You’re going to shut down the government if you can’t prevent millions of Americans from getting affordable care,” said Rep. Chris Van Hollen, D-Md.

A leader of the tea party Republicans, Sen. Ted Cruz, R-Texas, insisted the blame rests with Senate Democrats.

“The House has twice now voted to keep the government open. And if we have a shutdown, it will only be because when the Senate comes back, (Senate Majority Leader) Harry Reid says, ‘I refuse even to talk,”’ said Cruz, who led a 21-hour broadside against allowing the temporary funding bill to advance if stripped clean of a tea party-backed provision to derail ObamaCare. The effort failed.

The battle started with a House vote to pass the short-term funding bill with a provision that would have eliminated the federal dollars needed to put Obama’s health care overhaul into place.

The Senate voted along party lines to strip that out and sent the measure back to the House.