Win McNamee/AP Clinton did not disclose details of his plan, which faces Republican opposition

And objections to the plan raised today by Federal Reserve Chairman Alan Greenspan could make it that much harder for the idea to become a reality.

Clinton's proposal, unveiled Tuesday in his State of the Union address, must conquer worries about the wisdom of investing Americans' retirement money in potentially volatile stocks — especially amid persistent fears that stock prices may already be too high.

"Ultimately, I think it happens — but I think it happens with such a small sliver of the Social Security pie that it will not have a noteworthy effect on stocks in the near term," said Charles Crane, chief market strategist at Key Asset Management.

Clinton proposed using more than $2.7 trillion in expected budget surpluses over the next 15 years — or 62 percent of the total — to directly bolster Social Security's cash reserves. Of that, nearly $700 billion would be invested in the stock market by an independent government board in hopes of gaining higher returns.

The rest would be kept, as they are now, in safer but historically lower-yielding U.S. Treasury bonds.

But Greenspan, arguably the country's most influential monetary official, told the House Ways and Means Committee today he was concerned that investment decisions for Social Security would be influenced by political pressures.

Moreover, he said, the plan is bad for the economy. Social Security purchases of stocks would channel trillions of dollars over the years into U.S. companies; that would allow some industries to become less efficient because government action, and not market forces, are deciding which companies should be funded.

"I am fearful that we would use those assets in a way that would create a lower rate of return for Social Security recipients and even greater concern it would create sub-optimal use of capital and a lower standard of living," Greenspan said.

Also standing in the proposal's path are Republicans who oppose government participation in private financial markets, preferring to see Social Security money diverted into personal accounts. Clinton is siding with Democrats who say that would expose Americans to too much financial risk.

"So far the market regards this possible outcome with keen interest, but without tremendous conviction," said Tom Madden, chief investment officer for domestic equities at Federated Investors. "Given the president's current difficulties and the entire focus of the Congress on the impeachment trial, how fast such an initiative could go forward is an open question."

Fears also remain that stocks have climbed too fast and are too risky. Indeed, blue-chip stocks managed only a slight gain Tuesday as investors reacted to warnings from the Fed chairman that the market may be too optimistic about corporate profits.

Some analysts pointed out the appeal of the current system: it's safe.

"People are probably scratching their heads and saying the Social Security system ... is missing out on great investment opportunities by not having been in the market in the last eight years. But widows and orphans also haven't lost money either at any point," said Arthur Hogan, chief market analyst at Jeffries & Co.

"It's very difficult to explain to people: 'You lost money, you lost part of your Social Security, because we made bad investment decisions,"' Hogan said. "That can't happen in the boring old bonds that they're invested in now."

Clinton also proposed that another 11 percent of the total surplus, about $500 billion, would go to subsidize new accounts to supplement Social Security benefits that would be similar to the 401(k) plans many companies offer.

The Clinton plan promises to pour millions more into the market, but the more significant impact may be psychological, said Robbins.

"It adds a little more credibility to the stock market and may cause people to separately move more money from relatively low-return investments and realize that long-term stocks are the place to be," he said.