In a stunning turnaround, likely voters in the so-called millennial generation prefer a Republican-led Congress after next week's elections, and young Hispanics are turning sharply against President Obama.

A new national poll of 18-to-29-year-olds by Harvard's Institute of Politics shows that young Americans are leaving the new Democratic coalition that twice elected Obama. The news is little better for the GOP: These voters, who more than any other voting bloc represent the future of the American electorate, generally hold Republicans in the lowest regard.

The long-view IOP findings suggest that neither party is poised to win the largest generation in U.S. history—a pragmatic, demanding, relatively nonideological electorate raised in an age of terrorism, war, and government dysfunction.

"Millennials could be a critical swing vote," said IOP Director Maggie Williams, projecting the latest results on future elections. "Candidates for office: Ignore millennial voters at your peril." Williams is a Democrat and a former adviser to Hillary Clinton.

In the short term, the news is worse for Democrats than Republicans.

• Millennials who told the IOP they will "definitely be voting" Tuesday favored Republicans over Democrats, 51 percent to 47 percent. That is a reversal of September 2010 results, when the IOP found Democrats favored over Republicans among young likely voters, 55 percent to 43 percent.

• Obama's job-approval rating among millennials decreased from 47 percent in April to 43 percent, his second-lowest rating in the IOP surveys. Among young Americans most likely to vote, his job-approval rating is just 42 percent.

• Obama's job approval is below 40 percent on several issues, including the economy, health care, the federal budget deficit, and foreign policy. Nearly six of 10 young Americans disapprove of Obamacare.

• Among the one in four millennial voters who say they definitely will vote Tuesday, Republican-leaning constituencies are significantly more enthusiastic about the election than Democrats.

• Just 49 percent of young Hispanics approve of Obama's job performance, the lowest since IOP began tracking in 2009. That's a big drop from six months ago, when his rating among young Hispanics was 60 percent, and five years ago, when 81 percent of Hispanic millennials approved of Obama's performance. Only 17 percent of Hispanic youth plan to vote Tuesday, far smaller than the non-Hispanic percentages and likely a reflection of frustration over stalled immigration reform.

Disclosure: I'm a member of the IOP's senior advisory committee, a position that gives me an appreciation for the 26 millennial surveys produced since 2000. The latest KnowledgePanel survey, conducted with the Government and Academic research team of GfK, involved 2,029 18-to-29-year-old U.S. citizens between Sept. 26 and Oct. 9.

John Della Volpe, director of the IOP surveys, said the sweep of the work convinces him that Democrats and Republicans are losing the next generation. "Both parties should reintroduce themselves to young voters, empower them, and seek their participation in the upcoming 2016 campaign and beyond," he said.

For instance, millennials hate government gridlock. Asked on "whom do you place the most blame regarding the political gridlock in Washington," a whopping 56 percent said, "All of them."

Congressional Republicans were blamed by 22 percent, compared with 13 percent who blamed Obama and just 5 percent who blamed Democrats in Congress.

While the GOP holds the upper hand Tuesday among likely young voters, millennials overall are more inclined to approve of Democrats in Congress than Republicans, 35 percent to 23 percent.

Less than 10 percent of millennials identify themselves as tea-party supporters. Millennials narrowly favor Democrats over Republicans to handle the economy, immigration, foreign policy, race relations, and even health care.

Young voters traditionally split between the two major parties, as they did in 2000 and 2002. Two wars and Hurricane Katrina under an unpopular President George W. Bush drove millennials to Obama's promise of change and can-do bipartisanship. He didn't live up to his hype, and by 2013, many young voters were walking away from Obama and Democrats amid revelations about the administration's domestic spying programs and the botched launch of Obamacare.

The latest poll underscores Della Volpe's belief that "young people are returning back to pre-Obama roots of being a swing-vote constituency."