People wait in line to sign up for health insurance at an enrolment event in Commerce, California March 31, 2014. REUTERS/Lucy Nicholson

By Steve Holland and Susan Cornwell

WASHINGTON (Reuters) - Americans with health insurance under Obamacare, including Republicans, are generally satisfied with it, a Reuters/Ipsos opinion poll found, posing a quandary for Republican politicians who have long vowed to repeal it.

President Barack Obama's signature policy, the Affordable Care Act (ACA) of 2010, was opposed by 53 percent of almost 21,000 Americans surveyed, and favored by 47 percent.

But 60 percent of the roughly 1,800 survey respondents who have coverage through Obamacare favored the law.

Within that group, almost two-thirds were satisfied with the healthcare they were getting, including 73 percent of Democrats and 53 percent of Republicans.

"I'm happy to have it and I'm happy to get a little subsidy" to help pay for coverage said one Republican, Richard Rubenstein, 56, of Cincinnati, a Type 1 diabetic who was without health insurance for a year before signing up for a plan on the Obamacare federal website, HealthCare.gov.

One of the most divisive U.S. laws in decades, Obamacare helped launch the conservative Tea Party movement that has reshaped the Republican Party. But Republicans are now in a difficult position, with two powerful forces converging.

One is an effort by some congressional Republicans to repeal Obamacare, which is churning forward. The other is a Supreme Court ruling expected in June on a case brought by libertarian activists challenging the legality of federal Obamacare tax subsidies to help people afford coverage.

If the court invalidates these subsidies, as many as 7.8 million people in 34 states would lose them, and some of those people would be Republicans.

Most Americans were not aware of the Supreme Court case, known as King v. Burwell, the Reuters/Ipsos survey found.

But when asked about the subsidies at risk, 23 percent of those with Obamacare said they would support eliminating them and 43 percent said they would oppose it.

The poll found broad support among both parties for key tenets of Obamacare, such as banning insurers from canceling coverage if a person gets sick, or denying it for a pre-existing condition.

Eighty percent of Democrats and 63 percent of Republicans said they favored letting children stay on their parents' insurance until age 26.

Large majorities also favored Obamacare's expansion of the Medicaid health program for the poor and a requirement that companies with more than 50 employees should provide health insurance for their employees.

Many congressional Republicans are still pledging to repeal Obamacare, citing continued voter complaints.

"It hasn't died down," said Republican Senator James Lankford of Oklahoma. "It's still a very significant issue."

But there is increasing recognition among Republicans that any alternatives they might offer would need to include Obamacare's more popular features.

Senator Susan Collins, a moderate Maine Republican who voted against Obamacare, said Congress should fix the law, rather than try to repeal and replace it with a Republican system that Obama, a Democrat, would be certain to veto.

"Our time would be better spent on passing bills that fix the many significant flaws in the law, rather than passing a repeal bill that’s going to be vetoed," she said.

The poll underlined the deep divide on the so-called individual mandate requiring that all U.S. residents have health insurance.

Sixty-two percent of all Democrats favored the mandate, while 74 percent of all Republicans opposed it.

On the 2016 presidential campaign trail, there was no clear agreement among Republicans on the way forward. The single biggest item all Republicans use as fodder is that Obamacare represents the federal government run amok.

"We need to be as focused on the replacement as we are for the repeal" of Obamacare, former Florida Governor Jeb Bush told a conservative Club for Growth summit in Florida recently.

Republican pollster Whit Ayres said repealing the law and returning to what came before is "not really the issue. The issue is repealing it and replacing it with something that is substantially less onerous."

Should the Supreme Court end the subsidies, Republicans who control Congress could be propelled into action, likely with an effort to extend the subsidies temporarily.

The online survey was conducted from March 6-May 1. The poll's credibility interval, a measure of its precision, is plus or minus 0.8 percentage point.

(Additional reporting by Lawrence Hurley; Editing by Kevin Drawbaugh and Jonathan Oatis)