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.

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.”