WASHINGTON - The White House on Tuesday kicked off a six-month campaign to encourage millions of Americans to sign up for health coverage under "Obamacare," an effort in which the president and other political celebrities promote the law's promise of subsidized health coverage.

Former U.S. President Clinton shakes hands with U.S. President Obama after a discussion about healthcare at the CGI in New York. [Photo/Reuter]

But the massive public education campaign faces a long, difficult slog to persuade nearly 3 million healthy young people with low to moderate incomes to purchase private insurance. Many of them live in conservative, Republican-led states where opponents are spending millions of dollars to discourage enrollment in Obamacare's new, online health insurance marketplaces beginning October 1.

In promoting his signature legislative achievement on Tuesday, President Barack Obama sought to leverage his popularity among young adults by joining former President Bill Clinton - who also is popular with that age group - for a "conversation" in New York about healthcare.

First lady Michelle Obama and Vice President Joe Biden also are joining the Obamacare campaign, with separate appearances elsewhere. On Wednesday, former Secretary of State Hillary Clinton - who as first lady two decades ago led an unsuccessful attempt to revamp the U.S. healthcare system - will speak about healthcare at the annual meeting of the Clinton Global Initiative, her family's nonprofit foundation.

Next week, consumers in most states will begin to see more social media promotions from the Obama administration, targeting young adults in urban areas that are home to many of the nation's estimated 47 million uninsured people, according to senior administration officials.

The effort coincides with an expected $1 billion marketing initiative from health insurers, hospitals and health systems, as well as public outreach efforts by groups ranging from AARP, churches and charities to the Walgreen and CVS pharmacy chains, officials said.

"This is a Normandy invasion of the health system," said Uwe Reinhardt, a healthcare economist at Princeton University. "Eventually, lower-income people will be pleasantly surprised at how little health insurance will cost them with the subsidies."

Administration officials are confident that the Patient Protection and Affordable Care Act's initial six-month enrollment period, which runs through March 31, will meet its target of extending coverage to 7 million uninsured people, including 2.7 million adults aged 18-35 who are largely male and black or Latino. An estimated 33 million uninsured Americans could benefit from the program, officials say.

BIG SPENDING BY OBAMACARE FOES

The enrollment drive will have to overcome waves of ads from Republican, conservative and business groups that say Obamacare amounts to unwanted socialized medicine that will raise costs for businesses, eliminate thousands of jobs and force some people who already have health insurance to pay more for it.

Obamacare's critics already have launched a series of ads, ranging from sarcastic to fear inspiring, that are aimed at discouraging young adults from signing up.