After weeks of trailing Clinton in head-to-head polls, the billionaire businessman's numbers jumped after he effectively won the Republican nomination last week when his two remaining rivals quit, according to the online survey.

The national poll found 41 percent of likely voters supporting Clinton and 40 percent backing Trump, with 19 percent undecided. The survey of 1,289 people was conducted over five days and has a credibility interval of 3 percentage points.

The narrowing poll numbers are the result of both a gain by Trump and a drop by Clinton as she tries to overcome Democratic rival U.S. Senator Bernie Sanders.

The narrowing poll numbers are the result of both a gain by Trump and a drop by Clinton as she tries to overcome Democratic rival U.S. Senator Bernie Sanders.

While the general election campaign has hardly begun, the poll does mark a shift toward Trump. A similar Reuters/Ipsos survey conducted in the five days to May 4 showed the former secretary of state at 48 percent and the New York magnate at 35 percent.

With the party's primary seasons winding down, the two likely nominees have turned their attention to the Nov. 8 election and begun to test drive attacks - both on policy and personality - that will dominate the election for the coming months.

Clinton and Trump both poll well with voters of their respective parties, but independent voters continue to express uncertainty about who they will support, with 38 percent saying they are unsure or would vote for someone else.

But polls aside, one of the biggest factors in the race will be the changing demographics of the country as more minorities register to vote. Increased registration by Hispanics, who are likely to vote for Clinton, could play a roll in tipping the election her direction.

"This is an election that will be determined as much by the demographic composition of the American electorate as anything else - and that didn't change in a week," said University of Virginia political science professor Larry Sabato.

Clinton's loss in the Democratic primary election in West Virginia  signalled possible trouble for her in industrial states in November.

Trump, 69, has taunted Clinton in recent days, saying she "can't close the deal" against primary opponent Sanders, a U.S. senator from Vermont.

RYAN MEETING

But Trump still does not have some senior Republicans on board for his own campaign after primary election battles in which he promised to build a wall on the Mexican border to halt illegal immigration, and temporarily ban Muslims from entering the United States.

Several Republican leaders -- including House of Representatives Speaker Paul Ryan -- are withholding their support from Trump.

The former reality TV star will face pressure to tone down his rhetoric and clarify his policy positions when he visits Republican lawmakers on Thursday.

Trump may be considering an overhaul of his tax proposal, a move that could bring down the price tag and keep it in line with conservative ideologies.

A possible rework of the plan has been led by conservative economists Larry Kudlow, who hosts a program on CNBC, and Stephen Moore, who works with the conservative think tank Heritage Foundation.

"I have worked a little bit with Larry Kudlow to help try to tweak the plan to try to just... provide the maximum amount of economic juice and to reduce the cost," Moore told Reuters. "What we were working with the campaign a little bit on is how can we get that cost down, cut it by half or more, without disrupting the main growth elements of the plan."

Clinton took aim at Trump's tax plan at a rally in New Jersey on Wednesday.

With a typical American family earning $54,000 per year, Clinton said, "It would take that family 24 years of work to earn what Donald Trump's tax plan will hand out to people like him in just one year. That is no way to create good job with rising incomes for the vast majority of Americans, is it?"

For Clinton, 68, the West Virginia result underscored how she still needs to court working-class voters in the Rust Belt, including key states such as Ohio and Pennsylvania. West Virginia has one of the highest unemployment rates in country.

In West Virginia, roughly six in 10 voters said they were very worried about the direction of the U.S. economy in the next few years, according to a preliminary ABC News exit poll. The same proportion cited the economy and jobs as the most important issue.