Congress plunged the nation into a partial government shutdown Tuesday, forcing some 800,000 federal workers off the job as a protracted dispute over President Barack Obama's signature health care law reached the boiling point. Obama readied a midday statement to the nation while Democrats and Republicans at the Capitol blamed each other for the first shutdown in nearly two decades.

WATCH LIVE: President Obama's remarks on shutdown

"Closed" signs and barricades sprang up early Tuesday at the Lincoln Memorial, and national parks and many federal workplaces across the country followed suit. Agencies like NASA and the Environmental Protection Agency were virtually shuttered.

But people classified as essential government employees -- such as air traffic controllers, Border Patrol agents and most food inspectors -- will continue to work. So will members of the military and employees whose jobs are financed through fees, such as State Department workers who issue passports and visas.

With the Republican-controlled House and Democratic-controlled Senate stalemated, it was unclear how long the shutdown -- and the loss of some government programs and services -- could last. The Senate early Tuesday rejected the House's call to form a negotiating committee to resolve the deadlock.

The health care law itself was unaffected Tuesday as enrollment opened for millions of people shopping for medical insurance.

It was the first shutdown since a budget battle between Republicans in Congress and Democratic President Bill Clinton in the winter of 1995-1996.