By David Morgan

WASHINGTON (Reuters) - U.S. health insurance premiums have climbed faster than wages and inflation this year, and look poised to accelerate in 2013, adding to voter concerns about soaring healthcare costs ahead of November elections for the White House and Congress.

A study released on Tuesday showed that premiums for employer-sponsored health plans, which cover about 149 million Americans, grew a modest 4 percent to $15,745 in 2012. It was a substantially slower rate of growth than in past years, including 2011, when premiums jumped 9 percent.

But the study's authors at the nonpartisan Kaiser Family Foundation and the Health Research and Educational Trust, said higher costs still took a bigger bite from the income of middle-class employees, whose wages advanced only 1.7 percent, as employers shifted more healthcare costs to their workers.

This year's 4 percent increase eclipsed a general inflation rate of 2.3 percent. Some employers told researchers that insurers plan to push premiums up another 7 percent in 2013, the study said.

Rising costs could present a challenge for both President Barack Obama and Republican Mitt Romney in their battle for the White House. They also pose a dilemma for employers, which shoulder most of the cost and face the choice of absorbing ever-higher charges or making their workers pay more.

Polling data shows that soaring healthcare costs rank alongside the government's Medicare program for the elderly as a top campaign issue for voters.

The Obama administration is implementing healthcare reforms intended to rein in the cost of healthcare delivery, including provisions to restrain insurance premiums. But neither the president nor Romney has unveiled a comprehensive plan for controlling healthcare prices overall.

Just before the study was released, the Department of Health and Human Services announced that consumers have saved $2.1 billion on premiums under the Patient Protection and Affordable Care Act, which is better known to voters as "Obamacare."

The total includes $1.1 billion in rebates to 13 million consumers whose insurers overspent premium revenues on marketing, administration, bonuses and other costs not related to providing benefits. It was not clear how much of that money actually reached individuals versus their employers.

About 58 percent of employer-sponsored health plans are exempt from reform restrictions because they existed before the provisions became law in 2010 and have not altered benefits, deductibles or other charges significantly, according to the Kaiser study. The figure has fallen sharply from 72 percent last year as employers increasingly shifted coverage costs to workers.

Premiums for employer health plans have doubled over the past decade, with worker contributions surging, on average, to $4,316 from $2,137 in 2002, according to the study's January-to-May survey of 2,100 public and private-sector employers.

The growth in premium costs is roughly in line with broader health spending, which has moderated in recent years as a weak economy has prompted many people to forgo costly medical services including doctor visits.

"In tough times, when wages are flat, people avoid using the healthcare system if they can. We also know that higher out-of-pocket costs deter utilization," said Kaiser President Drew Altman.

But this year, the impact of the slow economy on insurance premiums also appears to have been magnified by employers' switching to lower-cost, high-deductible health plans that increase out-of-pocket expenses for workers. Employer contributions hit an average $11,429 this year, up from $5,866 in 2002.

The study also shed light on cost and benefit anomalies in the employer-sponsored insurance market, a pillar of the $2.8 trillion U.S. healthcare system since the 1950s that has begun to weaken after decades of uncontrolled cost increases.

About 61 percent of companies offer health benefits to their workers. But researchers found that workers at lower-wage companies pay $1,000 a year more for family coverage than workers at higher-wage companies, even though employers with large numbers of lower-wage workers pay less for the coverage they provide.

Lower-wage businesses are also more likely to offer plans with deductibles amounting to $1,000 a year or more.

Data also showed a 26 percent jump in the number of young adults receiving healthcare benefits on parental plans under healthcare reform, which extends childhood coverage to age 26.

There are now 2.9 million young adults on their parents' plans, up from 2.3 million in 2011, the study said.

(Reporting by David Morgan; Editing by Maureen Bavdek)