The results reflected a decline in support for Trump, rather than a boost for Clinton: In last Thursday’s poll, 42 percent of likely voters favored Clinton and about 39 percent favored Trump.

Among registered voters over the same period, Clinton held a lead of nearly 13 percentage points, up from about 5 percentage points on Thursday, according to the poll.

The five-day survey concluded on a mixed day for the Trump campaign. After squabbles last week with party leaders and the parents of a Muslim American soldier killed in Iraq, Trump sought to turn the page with a speech outlining an economic platform of tax breaks and regulatory rollbacks.

But in what was surely unwelcome news for Trump’s campaign, 50 heavyweight Republican national security officials, in a letter published on Monday, said that Trump would be “the most reckless president in American history.” Trump hit back, saying the signatories “deserve the blame for making the world such a dangerous place.”