In a letter on Tuesday to the A.F.L.-C.I.O., the Department of Labor said it
was ''very concerned'' that pension plans might be spending workers' money to ''advocate a particular result in the current Social Security debate.''

The Labor Department also warned the federation that pension plans could be
violating their fiduciary responsibilities by suggesting that they might take
their investment business away from Wall Street firms that support Mr. Bush's
plans.

The department did not cite any specific instances and it stopped short of
any formal accusations. But the letter came after a well-orchestrated campaign
by the A.F.L.-C.I.O. to criticize investment firms that appeared to be
supporting Mr. Bush's proposal for private investment accounts. ''A fiduciary
may never increase a plan's expenses, sacrifice the security of promised
benefits, or reduce the return on plan assets, in order to promote its views on
Social Security or any other broad policy issue,'' the letter said.

Damon Silvers, associate general counsel fo r the A.F.L.-C.I.O., described
the warning as mostly a matter of ''tone'' rather than substance, and said union
officials agreed with the Labor Department's main principles about fiduciary
responsibility.

''We don't view the substance of what the Department of Labor is saying as
alarming or threatening,'' Mr. Silvers said. ''The D.O.L. is saying that no
fiduciary can give business to an unqualified money manager based on the
politics that you like, and that's our position, too. We wouldn't want it any
other way.''

But House Republicans viewed the warning as a shot over the bow of labor at
a time when Mr. Bush's plan has largely stalled in Congress and when Wall Street firms have distanced themselves from it.

''The department confirmed that union leaders may not use their members'
pensions for purely political reasons to pressure employers into opposing Social
Security reform legislation,'' said Representative John A. Boehner of Ohio,
chairman of the Energy and Commerce Committee, adding that the administration should follow up with an investigation.

A.F.L.-C.I.O. officials organized a broad campaign this year against Wall
Street firms that appeared to support Mr. Bush's plan.

The campaign included public protests in 70 cities, and it focused on four
big companies that had joined a business-backed group called the Alliance for
Worker Retirement Security, which is lobbying for the private Social Security
accounts.

Two of the firms that were spotlighted, Edward Jones and Waddell & Reed,
dropped out of the lobbying group. The other two, Charles Schwab, the giant
discount brokerage company, and the Wachovia Corporation, stayed in the alliance but insisted they had no official position on Mr. Bush's ideas.

The financial services industry as a whole has been divided about whether to
get involved, even though many companies could benefit from Mr. Bush's proposal to let people divert part of their Social Security taxes to private investment accounts.

The Financial Services Forum, an industry association, dropped out of a
second advocacy group called Compass, which is trying to promote Mr. Bush's plan outside of Washington.

Derrick Max, executive director for both the Alliance for Worker Retirement
Security and for Compass, said the administration's warning could reduce the
pressure that union officials have tried to bring.

''They are very careful in how they word it, but their message is pretty
clear,'' said Mr. Max of the unions. ''I'd say it was the main reason that
Waddell & Reed dropped out, and it was a big reason that Edward Jones dropped out.''

The nation's labor unions run pension plans that hold hundreds of billions
of dollars in assets, and those plans are tightly regulated by federal
employee-retirement laws.

The A.F.L.-C.I.O.'s most visible campaigns against companies like Schwab and Edward Jones, which included public protests at brokerage firm offices in 70 cities, were not financed by the pension funds and were not themselves the
target of the government's warning.

The key issue in the warning was whether union-backed pension funds were
either spending money to protest Mr. Bush's proposal in communications with
pension-fund participants or threatening to take business away from qualified
investment firms that support the proposal.

Bill Patterson, head of the A.F.L.-C.I.O.'s campaign over investment
companies, said the government's warning would have little impact on his
efforts.

''We operate comfortably within the principles laid out by the Department of
Labor, and we're going to keep on doing what we've been doing,'' he said.