A Senate committee holds hearings today on a package of nominations to the Securities and Exchange Commission likely to alter the political balance of a body that has been dominated in recent years by an alliance of two Democrats and its retired Republican chairman.

Investor advocates yesterday sounded alarms about the nomination of Rep. Christopher Cox (R-Calif.) to be chairman, replacing William H. Donaldson, who rankled some lawmakers and trade groups by voting alongside the panel's two Democrats on several key rules. Cox's arrival would tilt the power at the agency firmly in favor of the Republicans, some legal analysts said.

Cox, 52, is a self-described free-market conservative who sponsored legislation that requires shareholder lawsuits to meet a higher burden of proof. He also opposed the treatment of stock options as an expense in corporate financial statements. The liberal advocacy group Public Citizen said yesterday that such moves should disqualify Cox to serve as SEC chairman.

But none of the lawmakers on the Banking Committee has publicly opposed Cox, in part because the White House is pairing his nomination with those of two Democrat-backed candidates. Sens. Paul S. Sarbanes (D-Md.) and Charles E. Schumer (D-N.Y.) said last week that they hoped all three nominees would be approved by the full Senate by the end of the month.

The Democratic nominees include Roel C. Campos, a member of the SEC since 2002. Campos, 56, a former federal prosecutor and telecommunications industry official, has been the less outspoken of the two Democrats on the commission. The other nominee, Annette L. Nazareth, serves as director of the SEC's market regulation unit, which oversees the stock markets.

She would replace outgoing Commissioner Harvey J. Goldschmid, a former general counsel at the agency who taught Nazareth when she attended Columbia University's law school. Goldschmid has been an outspoken and persuasive advocate, transforming Donaldson into a key ally despite their surface differences. Analysts said Goldschmid has been unusually effective because of his intellectual firepower and political skills.

Campos's first term expired in June. He took a lead role in the agency's outreach to international businesses after the passage of the 2002 Sarbanes-Oxley Act, which imposed new responsibilities on chief executives and auditors.

Nazareth, 49, joined the commission in the late 1990s as a lawyer to then-chairman Arthur Levitt. In her current role, Nazareth advanced a controversial plan to restructure the trading system. She is a former executive at Salomon Smith Barney and Lehman Brothers Inc. Her husband is Federal Reserve Board Vice Chairman Roger W. Ferguson Jr.

The nominations come at a time of profound unrest at the agency, which has fought to restore investor confidence after accounting failures at Enron Corp. and WorldCom Inc. In recent months, the five-member panel has been split by personal and philosophical disputes. Meanwhile, the U.S. Chamber of Commerce is pressing the SEC to relax some initiatives on the ground that the agency has overreached.

"There is no question the chairman will likely be the deciding vote and determine the direction of the commission as a voting body," AFL-CIO lawyer Damon A. Silvers told reporters in a conference call, urging senators to question Cox aggressively. "A majority of the commission that's bent on deregulating the markets could do enormous damage."

Joan Claybrook, president of Public Citizen, called Cox's record "atrocious from an investor point of view."

Both advocates predicted Cox would face tough questions from members of the Senate Banking Committee about his past congressional votes, as well as his views on business complaints that the SEC had gone too far.

Supporters of Cox cite his political acumen and his background as a Harvard-trained lawyer as reasons he should pass muster with the Senate. President Bush pointed to Cox's tenure as a corporate lawyer at Latham & Watkins LLP in nominating him last month to fill Donaldson's shoes. Senate aides said the nine-term congressman could face inquiries about his work in private practice in the 1980s, including his representation of First Pension Corp., whose founder ultimately served prison time for fraud. Cox has denied knowledge of the scheme.

White House spokeswoman Erin Healy said Cox would make an "outstanding" leader of the panel in an interview yesterday.

Cox would join an SEC that continues to grapple with a number of hot-button issues. Among them: the challenge of imposing suitable financial penalties against corporate lawbreakers, whether to forbid mutual fund board chairmen from having ties to management, curbing excessive executive compensation, and giving investors a role in nominating corporate board members.

Rep. Christopher Cox (R-Calif.), a former corporate lawyer, has been nominated as the next chairman of the five-member SEC. He would probably be a deciding voice on controversial issues in front of the agency.