Clinton leads Trump among swing voters...but not by much

Hillary Clinton's path to the White House will not be a "cakewalk" if she faces presumptive Republican presidential nominee Donald Trump in November, pollster and chief strategist for Clinton's 2008 campaign Mark Penn said Tuesday.

Clinton holds a 4 point lead among voters in swing states, according to a poll of 4,500 Americans designed by Penn and conducted in the last week of March by Harvard University's Center for American Political Studies. The former secretary of state leads Trump 45 percent to 41 percent among the crucial subset of voters.

Clinton leads Trump 46 percent to 40 percent among likely voters across the nation, the poll indicates. The margin of error in the poll is 1.5 percentage points.

"Hillary, on the one hand, I think has got to bring the country together. Trump has got to soften some of the divisiveness that he has been sowing in the political system if he hopes to get anywhere near 50 percent," Penn told CNBC's "Squawk Box."

The poll found what others have suggested: Both candidates are viewed unfavorably by the American public. Trump has a 60 percent unfavorable rating, while 52 percent of the electorate are negative on Clinton.

On the issues, Clinton has a "strong lead" on the economy, health care, and the rights of women and minorities, while Trump has a "significant edge" on national security and anti-terrorism, said Penn, who is managing partner at Stagwell Group.

The big message, said Penn, is that the country and parties are more divided than ever.

About 64 percent of Sen. Bernie Sanders' supporters said they would vote for Clinton in a general election. Among Republican voters who did not vote for Trump, 55 percent said they will cast their ballot for him in November.