Hillary Clinton and her campaign team will almost certainly use President Barack Obama's winning battle plan from 2008 and 2012 as a starting point for mapping out their own strategy for her fall campaign against Donald Trump. Clinton's hope would be to replicate Obama's success with the same constituency of "Obama coalition" voters that helped him to victory. But, Clinton's image with one crucial bloc of the Obama coalition -- young Americans aged 18-29 -- continues to slide to new lows. This could create real problems for her campaign if it does not change in the weeks ahead.

Gallup tracking over the past month (July 1-27) shows that 31% of 18- to 29-year-old Americans have a favorable view of Clinton, compared with 40% of 30- to 49-year-olds, 41% of 50- to 64-year-olds and 39% of those 65 and older. In other words, Clinton has the least favorable image among young Americans -- a flip from where it was one year ago when 47% of 18- to 29-year-olds said they had a favorable image of her, at the time higher than all of the older age cohorts.

The big shift in views of Clinton among this age group occurred in February and March of this year when young Americans' positive impression of Clinton fell below that of the other age groups, where they have stayed ever since. Clinton's image has dropped among all age groups but has fallen the most with young Americans.

In 2012, Obama won the youth vote over Mitt Romney by a margin of 67% to 30%. This strong performance was complemented by the higher-than-average turnout of 18- to 29-year-olds in both the 2008 and 2012 elections, providing Obama -- according to one independent analysis of the 2012 presidential election -- with the edge he needed to win the key swing states of Ohio, Florida and Virginia.

The years since the election have not dimmed Obama's star in the eyes of most young Americans. The July 1-27 sample shows that 64% of 18- to 29-year-olds approve of Obama's job performance, well above the other three age groups. A mid-July poll also revealed that 65% of this age group had a favorable opinion of Obama, 24 points higher than their favorable opinion of Clinton in the same poll.

Obama Job Approval, by Age 18-29 30-49 50-59 65+ % % % % Approve 64 51 44 41 Disapprove 30 45 52 56 July 1-27, 2016 Gallup

Clinton's popularity problem with young Americans stands out because young people skew Democratic in their political identification and are often disproportionately positive about national Democratic figures.

Nearly seven in 10 young Americans see Michelle Obama (69%) favorably, and another 64% view Bernie Sanders favorably; older Americans are less positive about both. The 55% of young people who have a favorable view of Bill Clinton exceeds his image among older Americans. Put more directly, while young Americans are the least likely of all age groups to have a favorable view of Hillary Clinton, they are the most likely to have a favorable view of other major Democratic figures, including her husband.

Hillary Clinton is also underperforming with young Americans relative to other Democratic Party presidential nominees in the past. In late July 2004, for instance, John Kerry enjoyed a 63% favorable rating with 18- to 29-year-olds. Kerry's favorable ratings with older Americans at the same time were all less positive: 57% among 30- to 49-year-olds, 56% among 50- to 64-year-olds and 52% among those 65 and older.

As noted previously, young Americans should be Clinton's natural constituency. Half identify or lean Democratic (50%), and 31% identify as liberal, the ideology most often associated with the Democratic Party. By contrast, all other age groups are less Democratic and more conservative.

Partisan and Ideological Composition of Age Groups 18-29 30-49 50-59 65+ % % % % Republican/Lean Republican 31 39 46 48 Democratic/Lean Democratic 50 42 41 40 Conservative 24 32 41 46 Moderate 39 38 34 30 Liberal 31 25 20 20 July 1-27, 2016 Gallup

In fact, Clinton's image with young people who identify or lean Democratic is nearly underwater. Among 18- to 29-year-olds who identify as Democrats, Clinton holds a 48% favorable rating and a 47% unfavorable rating. With all other Democrats, Clinton's favorable rating soars -- she has a 74% favorable rating with 30- to 49-year-olds, 82% with 50- to 64-year-olds and 83% with those 65 and older.

This week, Clinton was heartily endorsed by Bernie Sanders and Barack Obama, two of the most popular Democrats in the eyes of young Americans.

The Sanders endorsement is most critical, as it is almost certain that Clinton's poor image among young people is a direct result of their extraordinary liking for and recent support of the Vermont senator. There is the real possibility that former Sanders supporters, including many young adults, will come around to Clinton in the weeks ahead as it sinks in that that she is the Democratic nominee and that the alternative is Donald Trump. If young Americans do end up supporting Clinton by big margins -- as they have for past Democratic candidates -- it will likely be as much the result of voting against Trump as voting for Clinton. But whether that motivation can produce the turnout Obama was able to generate among young Americans in previous elections remains to be seen.