First Read is a morning briefing from Meet the Press and the NBC Political Unit on the day's most important political stories and why they matter.

Sanders supporters are slowly (but surely) coming around to Clinton

As Hillary Clinton campaigns Monday morning with Sen. Elizabeth Warren in Cincinnati, the newest NBC/WSJ poll finds that Bernie Sanders supporters are slowly -- but surely -- coming to Clinton’s side. In the poll, 45% of Sanders supporters have a positive view of the presumptive Democratic presidential nominee, while 33 percent have a negative view of her (+12). That’s an improvement from last month, when it was 38 percent positive/41 percent negative among Democratic primary voters backing Sanders (-3), but it’s still a far cry from the 60% positive/15% negative score that Clinton supporters have of Sanders (+45) in this new poll. What’s more, in the poll’s horserace result showing Clinton leading Donald Trump by five points, 78% of Sanders supporters are picking Clinton, while 10% back Trump and the rest are neither/other/unsure. But in a four-way ballot -- where Clinton’s lead is reduced to one point -- 63% of Sanders supporters are behind Clinton, 14% pick the Green Party’s Jill Stein, 9% back Trump, and 8% are for Libertarian Johnson. Bottom line: Clinton is a stronger position with Sanders supporters than she was a month ago, but she still has a ways to go. And that helps explain Warren’s appearance with Clinton as Sanders continues to hold off of an official endorsement.

Breaking down Clinton vs. Trump in the new NBC/WSJ poll

In the NBC/WSJ poll result showing Clinton ahead of Trump by five points, Clinton leads -- much like last month -- among African Americans (87%-5%), Latinos (69%-22%), those ages 18-34 (53%-30%), women (52%-35%). Trump, meanwhile, has the edge among whites (49%-37%), men (48%-38%) and independents (40%-30%). Additionally, Clinton gets overwhelming support from Democrats (85%-8%), and Trump gets similar, though slightly less, support from Republicans (79% to 9%). And keep this perspective in mind about our poll: In June 2012, President Obama was ahead of Mitt Romney by just three points, 47%-44%.

Explaining the difference between the NBC/WSJ and WaPo/ABC poll results

Our poll wasn’t the only survey that was released over the weekend. Also on Sunday, the Washington Post/ABC poll showed Clinton ahead by 12 points, 51%-39%, after Trump was up by two points in May. The biggest difference between the NBC/WSJ and WaPo/ABC polls? Our poll has Democrats with a four-point party ID advantage, while the WaPo/ABC poll has Democrats with a 12-point party ID edge. So when in doubt, average them out -- and in the latest RealClearPolitics average, Clinton leads Trump by nearly seven points in a head-to-head matchup. And remember, seven points was the final margin of the 2008 Obama-McCain contest.

Trump, Clinton remain the most unpopular presidential nominees in history of NBC/WSJ poll

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Fresh out this morning, our new NBC/WSJ poll also finds that Trump and Clinton remain the most UNPOPULAR presidential nominees/presumptive nominees in the survey’s history. Clinton’s positive/negative score stands at 33%/55% (-22), while Trump’s is even lower 29%/60% (-31). Here are the scores for the other politicians and organizations our NBC/WSJ poll tested -- from most positive to least positive:

FBI: 48% positive, 16% negative (+32)

Planned Parenthood: 48% positive, 29% negative (+19)

Bernie Sanders: 43% positive, 34% negative (+9)

Barack Obama: 48% positive, 41% negative (+7)

NRA: 42% positive, 36% negative (+6)

Paul Ryan: 27% positive, 29% negative (-2)

Democratic Party: 37% positive, 43% negative (-6)

Republican Party: 28% positive, 48% negative (-20)

Hillary Clinton: 33% positive, 55% negative (-22)

Donald Trump: 29% positive, 60% negative (-31)

The rest of the NBC/WSJ poll comes out later today.

Clinton and Warren to hit Trump on his Brexit response

Meanwhile, NBC’s Monica Alba reports that both Clinton and Warren are expected to criticize Trump’s response to Britain’s decision to leave the Europe Union. This comes after the Clinton campaign released a TV ad -- on national cable -- hitting Trump on this very topic. “Every president is tested by world events,” the ad’s narrator says. “But Donald Trump thinks about how his golf resort can profit from them,” using Trump’s quote that “When the pound goes down, more people are coming to Turnberry.” On “Meet the Press” yesterday, the Trump campaign’s Paul Manafort fired back at the ad. “The American people care about what is going to happen to their lives, about change. And the issues of Brexit, this kind of phony ad doesn't address those things. And Hillary Clinton is ignoring the reality because she's part of the establishment. She can't get away from the fact that she is part of the problem that's being rejected,” he said.

Here are your VP frontrunners

On NBCNews.com this morning, we’ve published articles looking at the Dem and GOP frontrunners to the vice-presidential running mates.

Top Tier

Dems: Tim Kaine, Elizabeth Warren, Sherrod Brown

GOP: John Thune, Bob Corker, Chris Christie, Rick Scott

In The Hunt

Dems: Cory Booker, Julian Castro, Tom Perez, Xavier Becerra

GOP: Mary Fallin, Mike Pence, Jeff Sessions, Newt Gingrich

On The Bubble

Dems: Al Franken, Shaun Donovan, Mike Mullen

GOP: Kelly Ayotte, Scott Brown

Veepstakes Watch

Tim Kaine weighed in on why his name’s in the mix – and whether he’s “boring” -- in an interview with one of us(!)… Former Iowa Gov. Tom Vilsack would be a safe choice for Clinton’s VP, but insiders worry he doesn’t have the personality for the job… The RNC is hoping to turn Sanders supporters away from Clinton’s pick, the Huffington Post writes... Elizabeth Warren is campaigning with Clinton for the first time in Ohio… Newt Gingrich says he has not heard from the Trump campaign but hypothesized that the candidate is “probably going to start thinking about it two days before Cleveland.”… Bob Corker praised Trump’s Scotland news conference as “one of his best events.” … Marco Rubio says it’s not a “viable option” to be Trump’s running mate.

On the trail

Hillary Clinton campaigns with Elizabeth Warren in Cincinnati at 10:30 am ET, and Clinton later travels to Chicago to speak at the Rainbow PUSH Int'l Women's luncheon. Don’t forget to check out the political unit’s rolling minute-to-minute coverage of all the latest 2016 developments at the On the Trail liveblog at NBCNews.com.