WASHINGTON -- Most Americans oppose funding U.S. President Donald Trump's wall along the U.S.-Mexico border and even many of his supporters reject his proposed budget cuts to scientific and medical research, according to poll released Thursday by The Associated Press-NORC Center for Public Affairs Research.

The president gets higher marks for efforts to boost defence spending and beef up the border patrol, the poll found.

The results come at a crucial time. Congressional Republicans and Democrats are wrestling over whether to include a down payment for the wall -- financed by U.S. taxpayers instead of Mexico, despite Trump's repeated promises -- in the spending bill to keep the government open at the end of the month. Trump has yet to pass a major bill through Congress as his 100th day in office nears.

The poll offers a mixed view of Trump's budget plan, which drew criticism from Republicans and Democrats on Capitol Hill. Americans supported cuts to foreign aid, but opposed Trump's planned cuts to spending on environmental programs and fighting climate change. People were more likely to oppose than favour cutting federal help for public broadcasting and the arts and by huge margins supported additional money for veterans' programs.

"The military is depleted, and the veterans have been hurting," said Margaret Hall, 77, a Trump supporter from Ft. Lauderdale, Florida.

Trump's spending plans are facing their first test on Capitol Hill, as lawmakers debate his request for $30 billion for an immediate cash infusion for the military and $3 billion for additional border security, including a $1 billion down payment for new fencing and other barriers along the U.S.-Mexico border.

The public opposes new spending for the wall, 58 per cent to 28 per cent. Nearly half say they strongly oppose funding for the project.

"I honestly think the wall is going to be a fiasco. If they want to get into the country, they're going to get into the country, regardless of what you put up there," said Wes Drought, 61, a firefighter and paramedic from Winnebago, Illinois, who strongly disapproves of Trump. "If you're going to do something, do it with the border patrol. If you want to create jobs, there's a job."

Democrats have come out strongly against the wall construction, and some Republicans are signalling they're not interested in a confrontation.

"I don't like the concept -- I don't think it's needed -- of a 2,000-mile wall as some envision," said Sen. Jeff Flake, R-Ariz., whose state is home to some of the most rugged terrain along the border. "It's just not needed. In some areas you just don't need it. In some areas you need wall, in some areas you need fences. In some areas you need surveillance."

Testifying on Wednesday, Homeland Security Secretary John Kelly said the wall won't be from "sea to shining sea," but in places where border agents say it would be most effective. He said the wall could extend beyond a physical barrier and include a mix of technology including drones.

Eighty-six per cent of Democrats and 57 per cent of independents oppose new spending for a border wall with Mexico. Republicans break about 2-1 in favour of the wall, but that's more narrow support than on other priorities.

The most popular proposal by far in the Trump budget is increasing spending on Veterans Affairs, which is supported by 74 per cent of Americans and opposed by just 8 per cent.

Americans are also more likely to favour than oppose increased spending on border patrol (50 per cent to 32 per cent) and decreased spending on foreign aid (50 per cent to 30 per cent). By a 47 per cent to 34 per cent margin, more also favour than oppose increasing spending on defence and the military.

But other pieces of the proposed Trump budget draw negative reviews. By a 64 per cent to 19 per cent margin, most oppose decreasing spending on scientific and medical research. That's a complaint echoed by Republicans and Democrats in Congress, including the Senate Majority Leader Mitch McConnell, R-Ky., who holds significant sway as Congress uses the president's proposal as a blueprint but writes the budget.

Most Americans also oppose cuts to spending on the environment and on climate change, by a 52 per cent to 28 per cent margin. Finally, Americans are more likely to oppose than favour cutting spending for public television, radio and the arts, 44 per cent to 32 per cent.

Large majorities across party lines favour increasing Veterans Affairs spending, although Republicans are particularly supportive. And decreasing spending for scientific and medical research is largely opposed across party lines, by 78 per cent of Democrats, 58 per cent of independents and 51 per cent of Republicans.

But there are large political divides on several other pieces of the budget proposal. Seventy-five per cent of Republicans but just 40 per cent of independents and 26 per cent of Democrats favour increased military spending. Eighty per cent of Republicans and 49 per cent of independents, but just 26 per cent of Democrats, favour increased border patrol spending. And 72 per cent of Republicans and 54 per cent of independents, but just 30 per cent of Democrats, favour decreased foreign aid spending.

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The AP-NORC poll of 1,110 adults was conducted March 23-27 using a sample drawn from NORC's probability-based AmeriSpeak panel, which is designed to be representative of the U.S. population. The margin of sampling error for all respondents is plus or minus 4.0 percentage points.

Interviews were conducted online and using landlines and cellphones.