Barack Obama grabbed the headlines. And Hillary Clinton unquestionably has the deepest roots on the issue. But the driving force in the Democrats' health-care primary to date has been third-place contender John Edwards--as even some of his rivals concede.

Sen. Obama, under pressure from critics who complain he's been all style and little substance thus far in the campaign, finally unveiled a detailed health-care proposal Tuesday. Introduced in Iowa, a state Edwards has worked hard in the early going, Obama offered a plan that is ambitious and expensive. He offeeds the uninsured a chance to buy affordable coverage similar to that enjoyed by federal employees. He promised to reduce the cost of insurance for a typical family by $2,500--through better use of technology, and by having the government pay for the most expensive catastrophic care. He would require all employers to make what he termed a meaningful contribution to their employees' coverage, while offering exemptions for small businesses. And he aimed to ensure that all children have coverage, and that young adults up to the age of 25 be eligible to continue with their parents' plan. The price tag: $65 billion a year, to be financed by rolling back the Bush tax cuts for the wealthiest Americans.

Obama's aides acknowledge that as they pieced together the proposal, they had their eye on Edwards--worrying about the scale of the former North Carolina senator's own package of health-care reforms, and how their offerings would be judged when the two plans were placed side by side. Edwards, whose plan resembles the Massachusetts model, calls for universal health care coverage, and he's clearly hoping that position will better endear him to Democratic primary voters; upon learning of the Obama plan, his aides faulted it for not going far enough.

Clinton, meanwhile, welcomed Obama to the health-care debate, as an old pro welcomes a rookie--even though she has yet to detail her own proposal for the 2008 campaign. "America is ready for universal health care," says a statement on the Clinton website. "Hillary has the vision and the experience to make it a reality." Yet the only glimpse of that vision to date was a speech at George Washington University last week that promised to focus on preventive health care and technological advances to lower costs. Clinton decided instead to spend Tuesday talking about income equality--another issue on which Edwards has staked out early ground (who can forget the "two Americas" mantra from his 04 campaign?). He may trail Obama by 20 points--and Clinton by 38 points--according to a recent sounding by the Pew Research Center. But if the domestic policy speeches of the frontrunners is any indication, Edwards is making his presence known.

Well, it's official, nonofficially: Fred Thompson is poised to jump in the race for the White House. The former Tennessee senator turned "Law & Order" actor will file paperwork next week to form an exploratory committee that would allow him to raise money for the run at the GOP nomination. The fine print is that because he's not officially a candidate, he can delay reporting how much money he's raised until next fall. Meanwhile, other White House hopefuls are required to report their second-quarter fund-raising totals to the Federal Election Commission by July 15. Why is this a big deal? For one, June is considered something of a do-or-die month for presidential wanna-bees who are rushing to raise the money they need to show they still have the momentum and stamina to stay relevant in the race for 2008.

Case study: John McCain, who had a less-than-thrilling first quarter financial showing, is doing money events almost every day this week to improve on or at least keep up with rivals Rudy Giuliani and Mitt Romney. Ditto for Sam Brownback and third-tier candidates, who need to raise enough to stay in the running. The plus for Thompson is that by forming an "exploratory" committee instead of an official campaign account, he can escape, or at least delay, the expectations game until Sept. 30, the third-quarter FEC deadline. Perhaps most important, Thompson can woo donors, including some who might have given to the other candidates.

How serious a threat do the other campaigns consider Thompson to be?