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 is 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 incomes 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.

The last government shutdown was 17 years ago.