By Andy Sullivan

WASHINGTON (Reuters) - President Barack Obama on Tuesday offered a campaign blueprint for fellow Democrats who are facing a tough election this November: Challenge Republicans who criticize his signature healthcare law to come up with something better.

The president had barely mentioned the healthcare law known as Obamacare during his previous two State of the Union addresses to the U.S. Congress.

This time, Obama gave a robust defense of his signature domestic policy accomplishment in what seemed to be an effort to give Democrats who are fighting to keep their seats some ammunition to counter a blitz of Republican attacks over their support of the healthcare law.

Obama made no mention of last fall's botched rollout of the healthcare program, or the disappointment of those who found that their insurance premiums had increased beyond what they could afford.

Instead, he portrayed the program - which is designed to give millions of uninsured Americans access to coverage - as a crucial safety net that protects people from financial and medical disaster.

Obama also said the Affordable Care Act fixes the injustices of a healthcare system that formerly allowed women to be charged higher rates than men and denied coverage to sick people.

"That's what health insurance reform is all about - the peace of mind that if misfortune strikes, you don't have to lose everything," he said.

His message could provide a playbook for Democratic senators such as Kay Hagan of North Carolina, Mary Landrieu of Louisiana, Mark Pryor of Arkansas and Mark Begich of Alaska as they try to convince voters in those Republican-leaning states that the law the senators supported is not the disaster that Republicans portray it to be.

Those races could determine which party controls the U.S. Senate in the final two years of Obama's presidency. Republicans need to pick up six seats to win control of the chamber; most analysts expect they will retain control of the House of Representatives.

The Democrats have their work cut out for them.

Public unease with Obamacare helped Republicans win control of the House of Representatives in 2010, and polls show the Affordable Care Act remains unpopular with voters - a fact Republicans aim to exploit again this year.

In North Carolina, Hagan is arguing that Republicans would take voters back to a time when insurance companies could drop people when they got sick.

She also has criticized Republicans in the state legislature for denying coverage to 300,000 people in the state by rejecting the law's expansion of the Medicaid health program for the poor.

By tackling the law directly, Obama likely made a better argument for his fellow Democrats than if he had sought to avoid it, said Robert Y. Shapiro, a Columbia University political science professor.

"If he doesn't talk about it then the Republicans are going to talk about it anyway. He has no power to keep it off the public agenda," Shapiro said.

REAL STORIES

In what has become a fixture of the annual presidential speech, Obama pointed to a "real person" sitting near his wife in the House of Representatives gallery - in this case, 37-year-old Amanda Shelley, a physician assistant from Arizona who would have faced bankruptcy from medical costs had she not gotten coverage under the law.

Republicans, of course, had plenty of "real people" of their own to highlight the law's shortcomings.

House Speaker John Boehner invited four business owners who he said were struggling because of the healthcare law. Other Republicans introduced a cancer survivor and business owners who said they had seen their costs go up because of Obamacare.

"It's because of these stories ... that we believe that many Democrats running for office will suffer the consequences of their actions in supporting Obamacare," Republican National Committee chairman Reince Priebus said.

Democrats have another line of attack as well.

House Republicans have voted more than 40 times to repeal Obamacare, but they have not coalesced behind alternative changes in the healthcare system. That has allowed Obama and his fellow Democrats to argue that Americans would lose many of the consumer protections included in their law if Republicans had their way - and to paint Republicans as being bereft of ideas.

"We all owe it to the American people to say what we're for, not just what we're against," Obama said.

OLDER VOTERS HAVE HEALTH COVERAGE

Still, it's not clear whether this confrontational approach will be enough.

U.S. presidents historically have seen their party lose seats in Congress two years after winning re-election. On top of that, older voters and white voters tend to make up a greater slice of the electorate in off-year elections, and those two groups tilt heavily Republican.

Older voters also are the least likely to see the benefits of the health law, as those over 65 already qualify for government-subsidized health benefits of their own through the Medicare program. For Democrats like Hagan, the best defense may be a good offense.

"If they run away and try to pretend they weren't there when this was happening, they are going to get slaughtered," said Democratic strategist Chris Kofinis.

(Additional reporting by David Morgan, John Whitesides and Susan Cornwell; Editing by David Lindsey and Jim Loney)