DETROIT — Four days before critical primary elections, Republican presidential contender Mitt Romney outlined a far-reaching plan Friday to gradually delay Americans’ eligibility for Medicare as well as Social Security.

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Romney said the shift, as people live longer, is needed to steer the giant benefit programs toward economic sustainability.

Speaking to the Detroit Economic Club — in cavernous Ford Field, where the Detroit Lions football team plays — he also made a play for primary election support in Michigan, which votes on Tuesday along with Arizona.

Romney said previous steps to toughen government emission standards had “provided a benefit to some of the foreign automakers” at the expense of American companies. He said future changes should be worked out cooperatively between government and industry.

Campaigning in the city where he was born, Romney described himself as “a car guy” who has a Ford Mustang and a Chevy pickup and whose wife, Ann, drives “a couple of Cadillacs.” Aides said they were model year 2007 and 2010 SRX vehicles, one each registered in Massachusetts and California.

Romney said his proposals for Medicare and Social Security would begin in 2022, meaning no current or near-retirees would be affected. He also said he favors adjustments to curtail the growth of future benefits for the relatively well-to-do, so “lower-income seniors would receive the most generous benefits.” He had described his Social Security proposals previously.

The two programs provide retirement and health care benefits to tens of millions of older Americans.

Beginning in 2022, Romney said, “we will gradually increase the Medicare eligibility age by one month each year. In the long run, the eligibility ages for both programs will be indexed to longevity so that they increase only as fast as life expectancy.”

Under current law, the age for collecting full Social Security benefits is gradually rising from 65 to 67. Medicare is available at age 65. In both cases, the age is set in law, and Romney’s suggestion that it be tied automatically to increases in the life expectancy of Americans would mark a major change.

He spoke in the run-up to a pair of primaries that mark his latest tests as he tries to break free of Rick Santorum and his other persistent but underfunded rivals in the presidential race.

He is widely expected to win Arizona. Neither he nor his rivals is airing television ads in the state, a reliable sign that all sides view it as a closed case.

Although public and private polls in Michigan show Romney has erased much or all of an earlier deficit, he still faces a stiff challenge from Santorum in the state, where the disparity in television advertising is not as overwhelmingly in Romney’s favor has it has been elsewhere.

It is an unwritten rule of Michigan politics that presidential candidates appear before the Detroit Economic Club. Santorum addressed the group several days ago, and officials familiar with the details said Newt Gingrich’s camp had been in discussions for an appearance as late as last week. A spokesman for the former House speaker did not respond to a request for comment.

Next: Romney Campaigns in Michigan.>>