Republican Donald Trump pulled even with Democratic rival Hillary Clinton in a Reuters/Ipsos opinion poll released on Wednesday, in a dramatic early sign that the Nov. 8 presidential election might be more hotly contested than first thought.

While much can change in the six months until the election, the results of the online survey are a red flag for the Clinton campaign that the billionaire's unorthodox bid for the White House cannot be brushed aside.

Trump's numbers surged after he effectively won the Republican nomination last week by knocking out his two remaining rivals, according to the poll.

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The national survey 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.

"Very happy to see these numbers," Trump said in a written comment to Reuters. "Good direction." A spokesman for Clinton's campaign did not respond to requests for comment on the poll.

A Reuters/Ipsos survey conducted in the five days to May 4 had the former secretary of state at 48 percent and the New York magnate at 35 percent.

Republican strategist Dave Carney said the Reuters/Ipsos poll showed the vulnerability of Clinton, who is still battling U.S. Senator from Vermont Bernie Sanders for the Democratic nomination.

“She has been in the public eye for decades, served in high office, and now she’s in a dead heat with Trump, in a race that everyone thought she would win easily,” said Carney, who has been critical of Trump. “Everyone thought it would be a romp.”

Trump has his own problems, though. He is struggling to bring some senior Republicans behind his campaign after primary election battles in which his fiery rhetoric rankled party elites.

Several Republican leaders – including House of Representatives Speaker Paul Ryan – are withholding their support.

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"After a tough primary, that's going to take some effort," Ryan said about unifying the party. "We are committed to putting that effort in."

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

Former Republican presidential nominee Mitt Romney criticized Trump on Wednesday for not releasing his tax returns, saying the only explanation was that the documents contained a "bombshell."

Trump has said that he will make public his tax returns on the completion of an audit.

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 in the Reuters/Ipsos poll saying they are unsure or would vote for someone else.

With the party's primary season winding down, the two likely nominees have turned their attention to attacking each other, both on policy and personality.

Clinton took aim at Trump's tax reform 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?"

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Trump has taunted Clinton in recent days for failing to "close the deal" against Sanders.

University of Virginia political science professor Larry Sabato said Trump – who has promised to force Mexico to pay for a border wall to halt illegal immigration and called for a temporary ban on Muslims entering the country – could also face a wall of opposition among minority voters.

"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," he said.

Clinton's loss in the Democratic primary election in West Virginia on Tuesday also signaled possible trouble for her in industrial states in November, underscoring how she still needs to court working-class voters in the Rust Belt.

Roughly six in 10 voters in West Virginia, which has one of the highest unemployment rates in country, 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 in the election.