Among those who said they know about the plan, 49 percent said they oppose the idea while 34 percent said they favor it, results released Wednesday indicated.
Pew said the results were basically unchanged from when the Republican-led House voted in favor of the voucher plan that was part of the budget proposal presented Wisconsin Rep. Ryan as chairman of the House Budget Committee.
The proposal would change Medicare into a program that would give future participants a credit toward buying private health insurance coverage.
The public also gave a relatively negative assessment of presumptive presidential nominee Mitt Romney's selection of Ryan as his running mate, Pew said. Nearly half -- 46 percent -- said Ryan is a fair or poor choice, while 28 percent said they thought he was an excellent or good choice.
However, Pew said results indicated the public views Vice President Joe Biden more negatively. Twenty-seven percent said Biden has done an excellent or good job as vice president, while 56 percent judged his performance as fair or poor.
Forty-two percent said they had heard a lot about Ryan, while 57 percent indicated they had heard little or nothing, Pew said.
Results are based on nationwide telephone interviews with 1,005 adults conducted Thursday through Monday. The margin of error is 3.6 percentage points.