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Paul Ryan is little known, was not in broad demand as Mitt Romney's VP pick and authored a budget plan that, while popular in the GOP base, stepped into the red zone on the subject of Medicare. His selection seems like an attempt both to rally Republican loyalists and to channel the public's substantial concern about the deficit. But given the sensitivity of Medicare, there is risk.

A few data points from recent and past polling:

-Ryan was one of seven Republicans bunched with single-digit preference for the VP nomination last month. Condoleezza Rice led with 30 percent, Marco Rubio had 12. (Poll from Fox News, 7/12)

-About as many Americans see Ryan favorably and unfavorably, 24 vs. 20 percent; most, 56 percent, either never heard of him (40 percent) or know of him but have no opinion (16 percent). (Poll from CNN, 6/12)

-Similarly, the public in June split by 23-17 percent on whether he'd be a good pick; 61 percent had no opinion. (Poll from Quinnipiac 6/12)

As Amy Walter notes , Democrats will use Ryan to try to cement their existing advantage in trust to handle Medicare. Obama leads Romney on the issue by 42-34 percent in mid-July. Testing the parties rather than the presidential candidates, it's the Democratic Party over the Republican Party by 40-24 percent in mid-June. (Polls from NBC/Wall Street Journal)

Further, asked in February if they trusted Obama "to make the right decisions" on Medicare, most Americans said they did, 58-41 percent. But most did not trust Romney on the issue; that was 42-52 percent (Poll from the Kaiser Family Foundation, 2/12).

Reaching back to Ryan's budget plan:

-At the time, Americans divided evenly on whose budget plan was better for the deficit: the Democratic plan proposed by Obama or the Republican plan proposed by Ryan, 44-43 percent. (Gallup 4/11) Without the party cues, just testing the Obama plan vs. the Ryan plan, it was 37-30. (Poll from Fox News, 4/11)

-In ABC News / Washington Post polling from 6/11, Americans opposed the Ryan budget plan by 50-32 percent; and the public preferred Medicare as it is, rather than as a voucher plan, by a broad 65-34 percent. Time hasn't changed that. In a Kaiser poll last February Americans opposed changing Medicare even more broadly, by 70-25 percent.

-Finally, at the time of the budget debate, Obama led the Republicans in Congress in trust to "protect the Medicare system" by a 15-point margin, 49-35 percent. (Again, ABC/Post 6/11)

Among related questions is whether the Medicare changes Ryan proposed may hamper Republican efforts to paint Obama's health care reform as unjustified social engineering. Many Americans are resistant to change in health care programs from either side of the aisle, given the uncertainties change implies. (Of course, Romney's own record on health care reform itself complicates this line of attack.)

Will it matter? In Kaiser's poll in February, 23 percent said Medicare is an issue on which they could only support a presidential candidate who shares their views. Sixty-two percent called it "just one of many issues," 14 percent, not an issue. Twenty-three percent is not an insubstantial number in this regard - but Romney, not Ryan, is the presidential candidate, and it's the top of the ticket that drives the vote.