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Americans split nearly evenly on Paul Ryan's selection as Mitt Romney's vice presidential running mate, a divided response that nonetheless represents a sharp gain for Ryan compared with the negative tilt that preceded his pick, a new ABC News/Washington Post poll has found.

In weekend interviews 38 percent responded favorably to Ryan joining the Republican ticket, up from 23 percent in pre-selection interviews last week. Positive views rose among independents as well as among Republicans, and among women. And Ryan was notably well-received among senior citizens - a group of interest given his plan to reshape Medicare.

See PDF with full results, charts and tables here.

At the same time, 33 percent of adults - disproportionately Democrats - continue to see Ryan's selection unfavorably, about the same after the announcement as it was before. The movement in his favor came among undecideds, 45 percent before the announcement, a still-substantial 30 percent after it. Enough are left to tilt the balance as they come to their judgment on the choice.

GROUPS - Sixty-two percent of Republicans respond positively to Romney's pick, 14 percentage points more than did so in the days leading up to the announcement. Many fewer independents see the choice positively, 39 percent, but that's up 20 points from its level last week. (Twenty-eight percent of independents respond unfavorably; a third are undecided.)

While Ryan is especially popular among conservatives, he has substantial room for improvement in this group: Fifty-four percent of conservatives have a positive view of his selection (up from 38 percent who responded favorably in advance); that peaks at 65 percent of those who call themselves "very conservative," vs. fewer than half, 46 percent, of "somewhat" conservatives.

Responses are more tepid in the center: Among moderates, 37 percent see the choice positively; as many, 41 percent, respond negatively. Nonetheless, positive responses among moderates are nearly double what they were before the announcement, with fewer undecided, so, as with the public overall, it's positive movement, albeit only to about a split decision.

That leaves liberals, a core Democratic group; they respond unfavorably rather than favorably to Ryan by a 41-point margin in this survey, produced for ABC by Langer Research Associates.

AGE and SEX - As noted, seniors are of interest given Ryan's proposal to revamp Medicare with a system in which the government would give older Americans a fixed sum with which to buy insurance. They moved in Ryan's favor, from a 28-28 percent favorable-unfavorable view prospectively to 46-28 percent this weekend. Again a sizable number, 26 percent, are undecided; debate over Ryan's position on Medicare may inform their views.

Younger adults, age 18-39, also moved in Ryan's favor, but from a more negative view before the announcement, 18-29 percent positive-negative, to a divided one, 34-36 percent.

Ryan also saw a gain among women, a weak group for Romney compared with Obama; favorable views of the choice rose from 19 percent of women last week to 37 percent this weekend. Among men they went from 27 percent previously to a similar 38 percent after the announcement was made.

This survey is based on two independent national samples consisting of 667 interviews Wednesday-Friday, before Ryan's selection was made public, and 522 interviews Saturday and Sunday, after its announcement.

Three other candidates were tested in the pre-announcement poll - Tim Pawlenty, Rob Portman and Marco Rubio - each receiving favorable-unfavorable scores roughly similar to Ryan's in the same period (21-30 percent for Pawlenty, 16-25 percent for Portman and 29-26 percent for Rubio, vs., as noted, 23-32 percent for Ryan.)

HISTORY - Initial views of a vice-presidential choice are not necessarily predictive of sentiments as the candidate becomes better known. Shortly after her selection for the GOP ticket in 2008, 58 percent of Americans expressed an overall favorable opinion of Sarah Palin. That turned out to be her high-water mark; positive views of Palin declined to 46 percent a week before the election, and lower subsequently.

This survey measured favorable or unfavorable views of Ryan as the vice presidential nominee. Polls in previous election years have used different measurements, including favorability overall, rather than specifically as the nominee; or support or opposition to the selection. Initial favorability ratings for some little-known figures have been quite low, e.g., 24 percent for Joe Lieberman and 29 percent for Dick Cheney in 2000, with six in 10, or more, expressing no opinion.

Another question, to be determined in the weeks ahead, is whether it makes a difference. It's axiomatic in national elections that the top of the ticket drives the vote. That said, just before the 2008 election, 44 percent of likely voters said McCain's choice of Palin had made them less likely to support him, vs. 18 percent more likely. And while vice presidential picks may not matter in most elections, in a close one - and Obama and Romney were at 47-47 percent among registered voters in an ABC/Post poll last month - everything can matter.

METHODOLOGY - This ABC News/Washington Post poll was conducted by landline and cell phone among two separate random national samples, one based on interviews conducted Aug. 8-10, 2012, among 667 adults, with a margin of sampling error of 4.5 points; and the second based on interviews conducted Aug. 11-12, 2012, among 522 adults, with an error margin of 5.5 points. The survey was produced for ABC News by Langer Research Associates of New York, N.Y., with sampling, data collection and tabulation by SSRS/Social Science Research Solutions of Media, Pa.

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