Most Americans continue to support legal abortion, as many oppose making it more difficult for abortion clinics to operate and two-thirds say the U.S. Constitution should trump state abortion laws. But views on legal time limits for abortions mark underlying ambivalence on the issue.

Fifty-five percent in this ABC News/Washington Post poll say abortion should be legal in all or most cases, while 41 percent believe it should be entirely or mostly illegal. Those almost exactly match their long-term averages, 55-42 percent, in more than 30 ABC/Post polls since 1995.

See PDF with full results and charts here.

A similar 54 percent oppose state laws that effectively make it more difficult for abortion clinics to operate (most of them "strongly" opposed), vs. 40 percent in support. And by more than a 2-1 margin, 66-30 percent, Americans prefer abortion laws to be decided constitutionally rather than by each state individually. Preference for federal over state abortion laws encompasses both 69 percent of those who support legal abortion and 63 percent of those who oppose it.

At the same time, reflecting underlying unease with abortion, more say it should be legal without limitation only up to 20 weeks (as in some recent state laws), as opposed to about 24 weeks, as the U.S. Supreme Court ruled in the landmark Roe v. Wade case; the division is 56-27 percent (an additional 8 percent volunteer that it should never be legal).

Supporters of legal abortion divide fairly closely on the 20- vs. 24-week issue, with 49 percent supporting the former and 42 percent favoring the latter. On the other hand, not surprisingly, preference for the shorter option rises to 69 percent among those who oppose legal abortion, with an additional 18 percent of them saying the procedure never should be legal.

GROUPS - Support for legal abortion, as usual, includes similar numbers of men and women (56 and 55 percent, respectively). Also as is typical, religious, partisan and ideological differences are sharp in this survey, produced for ABC by Langer Research Associates.

Among evangelical white Protestants, for example, 66 percent say abortion should be illegal in all or most cases; among non-evangelical white Protestants, an identical 66 percent say it should generally be legal. Catholics divide more evenly, 50-45 percent, legal-illegal. Among those who profess no religion, by contrast - one in six adults - 73 percent support legal abortion.

Sixty-five percent of Democrats and 58 percent of independents support legal abortion, while 58 percent of Republicans are opposed (partly because three in 10 Republicans are evangelical white Protestants). Seventy-nine percent of "very" conservatives oppose abortion in all or most cases, while 80 percent of liberals support it (partly, in this case, because nearly three in 10 liberals profess no religion).

Hispanics, a growing segment of the population (and a disproportionately Catholic group), divide narrowly on abortion, with 46 percent saying it should generally be legal, 50 percent, illegal. That compares with 57-40 percent among all non-Hispanics.

Differences also are evident in opinions on state legislation limiting the operation of abortion clinics, one of which recently was signed into law in Texas. Evangelical white Protestants support such laws while non-evangelical white Protestants oppose them, in each case by 33-point margins. Democrats, independents, liberals and moderates oppose these measures, by margins ranging from 14 to a very wide 58 percentage points, while Republicans, conservatives overall and "very" conservatives are in favor, by 23- to 44-point margins.

As noted, most prefer federal law to take precedence over state laws on the issue, though that preference is higher among Democrats than independents and among liberals and moderates vs. conservatives. Republicans and strong conservatives divide on whether state laws or the U.S. Constitution should carry the day.

Despite broad preference for federal rule, majorities in most groups also prefer a 20- rather than 24-week unrestricted period, from 51 and 55 percent of Democrats and independents, respectively, to about seven in 10 Republicans, strong conservatives and evangelical white Protestants. Liberals are closely divided on the question, even though about eight in 10 liberals support legal abortion, want federal law to prevail and oppose state measures that limit clinics' ability to operate.

In another example of the complexity of views on abortion, while similar numbers of men and women support legal abortion, women are more likely than men to oppose state laws restricting clinics (58 vs. 50 percent), but also are more apt to prefer a 20- to a 24-week unrestricted rule (60 vs. 53 percent).

METHODOLOGY - This ABC News/Washington Post poll was conducted by telephone July 18-21, 2013, in English and Spanish, among a random national sample of 1,002 adults, including landline and cell-phone-only respondents. Results have a margin of sampling error of 3.5 points, including design effect.

The survey was produced for ABC News by Langer Research Associates of New York, N.Y., with sampling, data collection and tabulation by Abt-SRBI of New York, N.Y