The poll released on Sunday shows Trump and Ms Harris tied in battleground states, where the former president held a five-point lead while Joe Biden remained in the race.
Ms Harris reportedly met her vetting team - led by former US attorney general Eric Holder - over the weekend and received in-depth presentations on their findings, including potential political vulnerabilities.
She met three of those candidates - Minnesota Governor Tim Walz, Arizona Senator Mark Kelly and Pennsylvania Governor Josh Shapiro - on Sunday.
She also had a meeting with another top contender - Transportation Secretary Pete Buttigieg - on Friday and is understood to have met several other candidates virtually, US media report.
Ahead of the decision, the Harris campaign was met with a flurry of lobbying efforts on behalf of - or criticising - the candidates.
Mr Shapiro, for example, has drawn sharp opposition from some left-wing groups for his support of private school vouchers in Pennsylvania - a Republican-backed proposal to send $100m to families for private school tuition and school supplies - as well as his pro-Israel views.
The announcement is expected to take place ahead of a Harris campaign rally in Philadelphia on Tuesday night.
The Philadelphia event will be followed by a string of campaign events across Wisconsin, Michigan, North Carolina, Georgia and Arizona, before ending with a campaign event in Las Vegas, Nevada, on 10 August.
Trump and his running mate, JD Vance, have both suggested that Ms Harris' choice of running mate will ultimately have no impact on the upcoming election.
In an episode of the "Full Send" podcast released on Friday, Mr Vance said that he believed that the vice-presidential choice "doesn't really matter, as much as this hits my ego".
" are going to vote primarily for Donald Trump or for Kamala Harris. That's the way these things go," he said.
Similarly, Trump has largely shrugged off questions Ms Harris's choice, arguing last week that the vice-presidential role "does not have any impact".
On Sunday, however, Trump criticised Mr Shapiro on Fox News, saying that Ms Harris could lose "her little Palestinian base" if she chooses him.
Mr Shapiro, who during his student days wrote in a college magazine that Palestinians were "too battle minded", told reporters on Friday that he now supported a two-state solution.
Presidential candidate Robert F Kennedy Jr says the 2014 incident that mystified New Yorkers was a prank.
More than 200 water rescues have been carried out since the weather system made landfall.
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