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Nearly two-thirds of Americans in a new ABC News/Washington Post poll oppose U.S. missile strikes against Syria, with momentum counter to the administration's proposed military action. Support rises if Congress were to authorize an attack, but only to about an even split.

As things stand now, 64 percent oppose air strikes, up by 5 percentage points from a week ago; just 30 percent are in favor, down by 6 points. If Congress rejects action, support drops to 17 percent, with 76 percent opposed.

See PDF with full results, charts and tables here.

It's a much closer call if Congress were to support air strikes, but even in this case the public divides essentially evenly on the issue, with 44 percent of Americans saying they'd favor air strikes, vs. 48 percent opposed.

This poll, produced for ABC by Langer Research Associates, finds increased partisanship as the administration has sought to make its case. On the basic measure, regardless of congressional action, 71 percent of Republicans oppose military strikes, up from 55 percent last week. Opposition likewise has increased among conservatives, from 59 percent last week to 74 percent now.

Regardless, air strikes remain opposed by majorities across the political and ideological spectrum. Even among liberal Democrats, a core support group for Obama, just 44 percent support military action, with 50 percent opposed. Across the political spectrum, support from conservative Republicans dives to 22 percent, with 73 percent opposed.

The increase in support if Congress approves of strikes is significant across political groups - 17 points among Democrats, 15 among independents and 12 among Republicans, to 58, 41 and 36 percent, respectively.

Barack Obama gets majority approval on one other aspect of the issue - sending the question to Congress. Fifty-seven percent approve of that decision.

But other results offer little help for the administration's position. Strength of sentiment, another key measure, continues to run against military action. Just 14 percent "strongly" favor the proposal, with 44 percent strongly opposed. Like opposition overall, strong opposition is up by 5 points from last week.

Beyond partisan and ideological groups, support for military action has lost 8 points since last week among men, to 35 percent; it's lower still, 25 percent, among women.

METHODOLOGY - This ABC News/Washington Post poll was conducted by landline and cell phone Sept. 4-8, 2013, in English and Spanish, among a random national sample of 1,020 adults. Results have a margin of sampling error of 3.5 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.