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Over the past year, Republicans turned the tables on entitlement programs, accusing Democrats of cutting hundreds of billions of dollars from Medicare in their health care overhaul.

Now it’s Democrats’ turn, and the focus is on Social Security.

At three campaign stops in Pennsylvania on Thursday, Republican Senate candidate Patrick J. Toomey faced people holding signs calling on him not to touch the program.

The official legislative version of Mr. Ryan’s plan stretches to 629 pages and touches on all areas: It would turn Medicare and Medicaid into vouchers to pay for private insurance, with spending increases tightly controlled; it would peg Social Security increases to slower-rising prices rather than wages, and would allow personal investment accounts as part of the program; and it would overhaul the tax code, removing most deductions and special carve-outs for individuals, and replacing the corporate income tax with a consumption tax.

As much as anything, Mr. Ryan’s plan has been the starting point of plenty of conversations among serious-minded legislators and think tanks.

The Center on Budget and Policy Priorities, a left-leaning think tank, said it amounts to “enormous tax cuts for the affluent” and challenged analysts who said it would reduce the debt, arguing that it instead would lead to decades of unsustainable debt levels. The Heritage Foundation, though, called the Ryan tax proposal “an intellectually sound, coherent and fundamental path to tax reform.”

Mr. Ryan has had dueling columns with New York Times columnist Paul Krugman, who said the congressman was “serving up leftovers from the 1990s, drenched in flimflam sauce.”

Along the way, he’s won praise from Mr. Obama and House Majority Leader Steny H. Hoyer, Maryland Democrat, for starting a discussion.

Mr. Hoyer said it’s striking that the plan, which Mr. Ryan first introduced in 2008 and then revised in January, hasn’t gained more Republican support.

“The Republican Party has run away from Paul Ryan’s plan, even though you’d expect it to rush to embrace a proposal based on spending cuts,” he said in a June speech calling for bipartisan solutions on long-term budget challenges.

Many of the HouseGOP’s strongest spending watchdogs are among the 13 co-sponsors of the bill, while a handful of Republican candidates in both the House and Senate have voiced varying levels of support.

“I like just about every part of it, and you’ve got to commend him for putting it out there,” Dan Kapanke, who is seeking to unseat Rep. Ron Kind, Wisconsin Democrat, said this month, according to the Tomah Journal.