As health-care legislation advances through Congress, the young adults who were so vital to President Obama's election are emerging as a significant beneficiary of his top domestic priority, but they are also likely to play a major role in funding any reform.

In a campaign-style rally Thursday at the University of Maryland at College Park, Obama will aim to tap his richest vein of support -- voters younger than 30 -- to help sell his reform plan to a more skeptical general public. "We're at an important turning point in our push for real reform," read the e-mailed invitation, "and it's critical that we seize this moment."

A 2008 study by the Urban Institute found that more than 10 million young adults ages 19 to 26 lack health insurance coverage. For many of those people, health-care reform would offer the promise of relatively inexpensive individual policies, which do not exist in many states today.

The trade-off is that young people would no longer be permitted to bet on their good health: All the reform legislation before Congress would require individuals to buy at least minimal coverage.

Another bill will be introduced Wednesday by the chairman of the Senate Finance Committee. Sen. Max Baucus (D-Mont.) will offer in it a proposal to keep premiums manageable: a bare-bones catastrophic policy that would protect young people from financial calamity while providing basic preventive care.

Drafting young adults into any health-care reform package is crucial to paying for it. As low-cost additions to insurance pools, young adults would help dilute the expense of covering older, sicker people. Depending on how Congress requires insurers to price their policies, this group could even wind up paying disproportionately hefty premiums -- effectively subsidizing coverage for their parents.

An array of Democratic senators continued to complain Tuesday about the affordability of reform, insisting that the final package should include much larger tax credits to help people cover the cost of insurance premiums.

"I want to make clear that in its current form I cannot put my support behind the Finance bill -- it will not have my vote," said Sen. John D. Rockefeller IV (D-W.Va.).

In part, young adults are uninsured because they are less likely to work for employers who offer coverage; they may not qualify for public programs such as Medicaid; and even the skimpiest private insurance plans may be too expensive alongside hefty student loan payments and credit card debt.

But some young people -- nicknamed the "young invincibles" -- are also likelier than other Americans to assume that they won't need health insurance or to decide that they'd rather spend their money on other things.

To discourage that attitude, the Finance Committee bill would fine individuals who do not purchase coverage. An early draft of the proposal set the penalty at $750 or $950 per year for single people, depending on income. But according to various insurance experts, even the least expensive plan under the bill could cost more than $100 per month, making it cheaper for people to pay the fine than to buy insurance.

All the bills seek to blunt the additional cost to young adults, mainly through subsidies, but it is not clear what effect that would have. "The primary question is what the premium is and what people get for that," said Mark McClellan, director of the Engelberg Center for Health Care Reform at the Brookings Institution and a former senior Bush administration official.

Adding preventative care to a catastrophic policy makes the Finance Committee bill's bare-bones coverage more appealing, McClellan noted. But for many young adults, health care will become a significant new expense. "It's important for people to know what they're getting into," he said.

But it's also essential that young, healthy people participate, said Linda J. Blumberg, a health-care expert at the Urban Institute, because the requirement that people have insurance "is really a mechanism for financing health-care reform."

The more people steered into the system through such a mandate, Blumberg and others explained, the lower the total subsidies that the government must provide to keep insurance affordable. But if young people slip through the cracks -- or if Congress, facing political pressure, provides generous exemptions from the mandate -- then the government and people who buy coverage will face higher costs.

The Finance Committee proposal also focuses on broadening access to health insurance. For uninsured people without affordable employer coverage, it would open new insurance "exchanges," offering a menu of options at different cost and benefit levels. Additionally, all adults with incomes below 133 percent of the federal poverty level -- or about $14,400 for an individual -- would be eligible for Medicaid.

One group that policy experts worry could be squeezed by reform is young adults with health problems, whose incomes are not high enough to afford the expensive policies they may need to manage chronic conditions. Blumberg said young adults with asthma, diabetes, hay fever and even high school sports injuries are systematically rejected by insurers in states without protections for people with preexisting conditions.

Krisja Hendricks, 28, is a waitress in Brooklyn whose thyroid cancer was diagnosed shortly before she graduated from college, while she was still covered under her father's plan. Crohn's disease was later diagnosed, causing insurers to turn her away. She finally found a health plan for $245 per month, but she just discovered that it will not cover the tests she needs to monitor her health. "I'm willing to pay $400 a month, even though that's a lot," she said. "But I know I have to. I really don't think everyone should required to."

According to a Washington Post-ABC News poll last week, young adults are more optimistic about the outcome of health-care reform than those age 30 and older, but they are evenly divided on the cost implications, with 32 percent expecting their costs to decline and 27 percent expecting an increase.

About 52 percent of young adults support the idea of the individual mandate, about the same proportion as in other age groups. But in terms of the overall package, the under-30 group broadly supports the Democratic effort, with 60 percent favoring the proposed reforms vs. 42 percent among older adults.

And while the number is down from its high point, 63 percent of under-30s approve of Obama's overall job performance, significantly more than in other age groups.

Given the implications of reform, advocates for young voters wonder why they haven't commanded special attention from the White House and Congress, as have seniors, union households and industry stakeholders.

"We can do our part, but we need to hear from the people who are making the policy decisions," said Heather Smith, executive director of Rock the Vote, a nonprofit group aimed at drawing young people into the political process.

Along with other pro-reform organizations, Rock the Vote has begun a national advertising and grass-roots campaign to educate young adults about the emerging legislation. But Smith said she was frustrated that Obama offered few assurances to young adults in his speech before Congress last week, instead chastising as "irresponsible" those who don't buy coverage.

The under-30 crowd remains by far the president's most loyal following, Smith noted. "He needs to talk to them," she said. "Writ large, they are struggling; they are the uninsured."