Republicans may not be able to rely on social issues as they try to rebound against a popular president and increasingly dominant Democratic Party, as a new Washington Post-ABC News poll shows rising support for same-sex marriage, legalized marijuana and a process by which undocumented immigrants could become legal residents.

None of these issues has become an early priority for President Obama, who has marshalled his political capital on the economic stimulus package and revamping foreign policy instead. He does not support legalizing marijuana, and has said he favors civil unions giving gay couples the same legal rights as married couples, though not marriage.

Obama supports a path to citizenship for some illegal immigrants, but in last night's press conference, the president emphasized the need to secure the border ahead of comprehensive immigration reform and avoided putting a specific timetable on passing such a law.

Support for each of these issues has reached a new high in Post-ABC polling, but none has the widespread backing necessary to avoid a political battle. By keeping his focus elsewhere, Obama may be avoiding a repeat of former president Bill Clinton's failed efforts to end the ban on gays in the military....

But the shifting views in the new poll suggest Republicans face a greater hazard: An inability to use those issues to rally their base and appeal to conservative Democrats and independents who previously would have been put off by Democrats' more liberal stances on social issues.

Most striking is the sharp shift in public opinion on same-sex marriage. Forty-nine percent said it should be legal for gay people to marry, and 46 percent said it should be illegal. About three years ago, a broad majority said such unions should be illegal (58 percent illegal to 36 percent legal)....

Steve Schmidt, the Republican strategist who managed John McCain's campaign, recently came out strongly in favor of gay marriage and warned that his party risked being marginalized on the issue.

The poll results back up Schmidt's warnings, as they show shifts in opinion among the swing voters the GOP needs to woo -- independents, white Catholics and the young more broadly. Among independents, there has been a nine-point increase in support for legal gay marriages since 2006, to 52 percent, with strong opposition dropping 10 points over that period.....

The poll showed just how much of the movement is occuring among younger voters. Support for gay marriage has grown somewhat among voters over age 65, from 15 percent to 28 percent, but six in 10 remain strongly opposed. Among those under 35, though, two-thirds support it, up from 53 percent in 2006, and nearly half support it strongly. More broadly, Republicans face a sharp partisan gap among those under 35, only 17 percent identified as Republican, with 43 percent calling themselves independent and 36 percent Democrats. Across all age groups, 21 percent identified as Republicans in the poll, the lowest level in Post-ABC polling since 1983.....

While 74 percent said that the government is not doing enough to keep illegal immigrants from entering the country, including majorities across party and demographic lines, public sentiment has shifted away from that view since its peak in 2007 among most groups outside the Republican core. At the same time, a growing slice of the population -- including Republicans -- support the broad outlines of a plan giving illegal immigrants "the right to live here legally if they pay a fine and meet other requirements." Six in 10 overall (61 percent) would support such a plan, up from 52 percent in June 2007. Support is highest among younger voters, and independents have gone from being roughly split on the plan to being mostly in favor of it, 59 percent now support it.

Respondents were near split on another issue that until recently was deemed untouchable in many parts of the coutry -- marijuana legalization. Forty-six percent of all respondents said they supported legalizing "possession of small amounts for personal use," with rates of support higher among men, among younger voters and among independents, a majority of whom supported legalization.

One issue in the survey where public opinion has drifted in the conservative direction is gun control, where Obama and other Democrats have studiously avoided pushing for major reforms. Fifty-one percent of respondents said they favored stricter gun laws, down 10 points from 2006.

And fewer now say stronger laws would reduce the amount of violent crime in the U.S., and six in 10 said enforcing current gun laws is a better way to diminish crime than passing harsher laws.