WASHINGTON - Support for the war in Afghanistan has hit a new low and is on par with support for the Vietnam War in the early 1970s, a bad sign for President Obama as he argues that to end the war responsibly the United States must remain in Afghanistan another two years.

Only 27 percent of Americans say they back the war effort, and 66 percent oppose the war, according to an AP-GfK poll released Wednesday.

A November 1971 Harris poll showed a record-high 65 percent of Americans said that continued fighting in Vietnam was "morally wrong." By that time the United States was already drastically cutting the size of its force in Vietnam on the road to a full withdrawal in 1973.

The context is very different now, without a military draft that fueled the Vietnam War's unpopularity and with a comparatively small force.

The 11-year Afghan war has not been highly popular among Americans for many years, but support has dropped off steeply. A year ago, 37 percent favored the war, and in the spring of 2010, support was at 46 percent.

The AP poll does not spell out why people have changed their minds. But the drop-off in support parallels rising casualties, increased attacks on Americans by the Afghan soldiers they are mentoring and inconclusive battlefield gains that have increased security in many areas of the country but have failed to break the Taliban-led insurgency.

About half of those who oppose the war said the continued presence of American troops in Afghanistan is doing more harm than good.

Chris Solomon, an independent from Fuquay-Varina, N.C., is among the respondents who strongly oppose the war. He said the military mission has reached the limits of its ability to help Afghans or make Americans any safer, and he would close down the war immediately.

While the rationale for the war is to fight al-Qaeda, most of the combat is against an entrenched Taliban insurgency that will outlast the foreign fighters, Solomon said. He said the conflict is reminiscent of Vietnam.

"What are we really doing there? Who are we helping?" he said in an interview.

The Associated Press-GfK Poll was conducted May 3-7 by GfK Roper Public Affairs & Corporate Communications. It involved landline and cellphone interviews with 1,004 adults nationwide and has a margin of sampling error of plus or minus 3.9 percentage points.