President Obama delivered a staunch defense of the Affordable Care Act on the week of its fifth anniversary Wednesday as he continues his bid to frame the health care law as a success in the face of legal and political challenges from Republicans.

"It's working despite countless attempts to repeal, undermine, defame and defund it," Obama said during an event at the White House announcing a new network to help implement the law. "We've been promised a lot the past five years that didn't turn out to be the case: death panels, doom, a serious alternative from Republicans in Congress."

Obama also appeared to endorse a budding bipartisan agreement between House leaders that would permanently change a funding formula for payments to physicians who treat Medicare patients, replacing a system that was created in the 1990s but was regularly "fixed" because Congress did not want to cut payments to doctors with elderly patients. Some Senate Democrats have expressed concerns with elements of the plan.

"As we speak, Congress is working to fix the Medicare physician payment system," Obama said. "I've got my pen ready to sign a good, bipartisan bill, which would be really exciting. I love it when Congress passes bipartisan bills I can sign. It's always very encouraging."

Pedro Rojas holds a sign directing people to an insurance company where they can sign up for the Affordable Care Act, also known as Obamacare, on Feb. 5, 2015, in Miami. (Photo by Joe Raedle/Getty Images)

More than 16 million people who were previously uninsured now have medical coverage under the law, which is generally viewed as the president's signature legislative achievement. But the law, passed by Congress in 2010, is facing a Supreme Court ruling this spring that could wipe out subsidies for millions of them. The administration has not announced a backup if the court rules portions of the law unconstitutional.

"Kicking millions off insurance will somehow make us more free?" Obama asked incredulously.

Republicans, meanwhile, have continued their political assault, including Sen. Ted Cruz (R-Tex.), who has been one of the loudest critics and announced this week he will run for the GOP nomination for the White House in 2016. Cruz said this week that although he remains committed to repealing the law, he and his family would sign up for coverage on the federal exchanges because his wife will take a leave from her job at Goldman Sachs when she helps his campaign.

But Obama mocked the GOP's continued focus on overturning the law, challenging them to deliver on promises to develop a legitimate alternative. He said conservatives had developed many of the tenets of his plan, noting the former Massachusetts Gov. Mitt Romney had implemented a version of it in his home state before denouncing Obama's plan during their contest for the White House in 2012.

"If they want to take credit for this law, I'm happy to share it," Obama said. "Health care reform is hard. ... we've made our share of mistakes since we passed this law, but the policy has worked."