Hillary Clinton expands her lead over Donald Trump in national polls, but battleground states still show close races. Polling suggests Bernie Sanders would be the most helpful vice presidential pick for the Democrats. And demographic breakdowns of Brexit voters show big splits by age and party. This is HuffPollster for Monday, June 27, 2016.

A TALE OF TWO NATIONAL POLLS - HuffPollster: "Two new polls released on Sunday both show Trump on the decline from one month ago and trailing Hillary Clinton, the presumptive Democratic presidential nominee, by a significant margin. An ABC/Washington Post poll finds Trump trailing Clinton by a remarkable 12 points. In that poll, 51 percent of voters said they would vote for Clinton while 39 percent said they would vote for Trump. The poll shows an astonishing 14-point swing from one month ago — Trump has fallen by 7 points while Clinton has gained by the same amount. Last month, Trump had a slight edge on Clinton in the ABC/Post poll, with 46 percent to her 44 percent of the vote. An NBC/Wall Street Journal poll also released Sunday finds Trump trailing Clinton by 5 points. Clinton carries 46 percent of support in this poll, while the real estate mogul takes 41 percent. The NBC/WSJ poll shows just a 2-point decline for Trump from one month ago." [HuffPost]

Trump’s support is steady; Clinton’s differs between the polls - The main difference between Clinton’s 12-point lead in the ABC/Washington Post poll and her 5-point lead in the NBC/Wall Street Journal poll is how much support she gets. The difference in Clinton’s numbers -- 51 percent compared to 46 percent -- is outside the polls’ margins of error. But that doesn’t mean there’s something wrong with either poll. Margins of error only provide an estimate of sampling error, not every possible way a poll could differ. In the NBC/Wall Street Journal poll, which has Clinton at 46 percent, 13 percent of respondents chose options other than Trump or Clinton. In the ABC/Washington Post poll, 10 percent opted for a response that wasn’t one of the two major candidates. That could easily be attributed to different interviewer practices -- how much interviewers are instructed to push respondents for an answer -- and varying answer options. Additionally, the ABC/Washington Post poll had a more Democratic-leaning sample than the NBC/Wall Street Journal poll. HuffPost Pollster’s average falls in the middle of the two surveys, giving Clinton a 7-point lead over Trump.

Sanders supporters are boosting Clinton - Aaron Blake: "A new Washington Post-ABC News poll shows that Sanders backers, who polls have shown were reluctant to jump over to Clinton and even flirted with supporting Trump, are coming home faster than we might have expected. Last month, 20 percent of Sanders supporters said they would back Trump over Clinton in the general election. This month, that figure is down to 8 percent. And the poll was conducted before, we would note, Sanders began saying last week that he would support Clinton over Trump in the general election….What's more, the 81 percent of Sanders backers who are now behind Clinton is a higher number than in any poll of 2008 Clinton backers who rallied to Obama. The high that year was 74 percent, in October." [WashPost]

THE RACE IS CLOSER IN SOME BATTLEGROUND STATES - Anthony Salvanto: "Hillary Clinton holds narrow leads over Donald Trump across a number of key states of Florida (up three points, 44 to 41 percent); Colorado (Clinton 40 percent, Trump 39 percent); Wisconsin (Clinton up 41 percent to 36 percent) and North Carolina, which has flipped back and forth between the parties in the last two elections, where it's Clinton 44 percent and Trump 42 percent….Trump is also competitive in large part because of partisanship, as rank-and file Republicans continue to get behind him, even as Republican leaders have been more lukewarm toward the way Trump is running his campaign….Partisanship is driving much of these horse races too. Despite the hard-fought primary contests on both sides, Democrats in these states are now lining up behind Hillary Clinton and Republicans behind Trump--each garnering around eight in ten from their respective camps. And much of the vote appears locked in already: the bulk of those not voting for Clinton say they will not consider her, and the bulk of those not voting for Trump say they will not consider him." [CBS]

Florida could be easier for Democrats than Pennsylvania - Domenico Montanaro: "Trump continues to be competitive in places like Ohio and Pennsylvania because of blue-collar white voters….Because of demographics, Florida has appeared to us to be, if not leaning, moving toward Democrats, especially with Trump on the ticket....A Quinnipiac poll this month showed Clinton up 8 (47 to 39 percent), though she only leads by 3 in the RCP average. Of course, while the fundamentals appear to favor Clinton there, Obama won it by less than a point in 2012 and Democrats worry that strict Voter ID laws could make it tight. Democrats have won Pennsylvania in every presidential election in the last quarter-century (since 1992). But Pennsylvania is a place that is an emerging battleground." [NPR]

BERNIE SANDERS AS VICE PRESIDENT COULD BOOST THE DEMOCRATIC TICKET - Samantha Neal: "Thirty-nine percent of voters nationwide said they would be more likely to vote for the Democratic ticket if Vermont Sen. Bernie Sanders were on it, according to a new poll from Monmouth University. The poll tested six potential Democratic and six potential Republican vice presidential picks, and Sanders was the only name to stand out from the pack. Among undecided voters, a whopping 50 percent said they would be more likely to support the Democrats if it’s a Clinton-Sanders ticket. That could be a substantial number of voters — the HuffPollster model indicates that 9.8 percent of voters are undecided. Although voters would like Sanders as the vice presidential nominee, the Clinton campaign reportedly does not view a Clinton-Sanders ticket as an option….[Elizabeth] Warren is the second-place finisher among vice presidential options — 24 percent of voters said they would be more likely to vote Democratic if Warren were on the ticket…[Tim]Kaine mobilizes a low level of voter interest...Only 9 percent of all voters nationwide say they would be more likely to vote Democratic with Kaine on the ticket." [HuffPost]

BREXIT VOTERS SPLIT BY AGE AND PARTY - Michael Ashcroft: "The UK has voted to leave the European Union. On referendum day I surveyed 12,369 people after they had voted to help explain the result – who voted for which outcome, and what lay behind their decision…. The older the voters, the more likely they were to have voted to leave the EU. Nearly three quarters (73%) of 18 to 24 year-olds voted to remain, falling to under two thirds (62%) among 35-44s. A majority of those aged over 45 voted to leave, rising to 60% of those aged 65 or over. Most people with children aged ten or under voted to remain; most of those with children aged 11 or older voted to leave….A majority of those who backed the Conservative in 2015 voted to leave the EU (58%), as did more than 19 out of 20 UKIP supporters. Nearly two thirds of Labour and SNP voters (63% and 64%), seven in ten Liberal Democrats and three quarters of Greens, voted to remain." [Lord Ashcroft Polls]

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MONDAY'S 'OUTLIERS' - Links to the best of news at the intersection of polling, politics and political data:

-Donald Trump and Hillary Clinton are even less popular than controversial institutions like the NRA and Planned Parenthood. [NBC]

-Philip Bump finds that Trump supporters are more certain they'll vote than Clinton supporters are. [WashPost]

-Sixty-six percent of Americans say Trump's views on women and Muslims are biased. [ABC]

-Obama's approval rating hits 52 percent in the CNN/ORC poll, outranking both George W. Bush and Ronald Reagan at this point in their second terms. [CNN]

-American voters stand divided on the government's role in gun regulation. [NBC]

-Michael Coren explains why one mobile app pollster was able to accurately predict the Brexit vote. [Quartz]

-The Economist argues that betting markets are not proving more reliable than polls in recent elections. [Economist]

-Claire Durand, president elect of the World Association for Public Opinion Research, explains what went wrong with the Brexit vote. [Ah! Les Sondages]

-Maggie Koerth-Baker explains why political scientists are less likely to study what makes people more conservative. [538]