Partly it is her ubiquity, the guiding hand in everything from who sits on the Supreme Court to who sits next to whom at state dinners, the White House staff memos peppered with “VJ thinks” or “VJ says.” When the billionaire investor Warren E. Buffett showed up for a private lunch with the president last July, the table was set for three.

Ms. Jarrett often serves as a counterweight to the more centrist Clinton veterans in the administration, reminding them and her innately cautious boss that he came to Washington to do big things. Some of his boldest moves, on women’s issues, gay rights and immigration, have been in areas she cares about most. If Karl Rove was known as George W. Bush’s political brain, Ms. Jarrett is Mr. Obama’s spine.

She is also his gatekeeper, sometimes using that power to tip the balance in internal debates. After the financial crisis, as the administration grappled with how to rein in Wall Street, Ms. Jarrett made sure that Paul A. Volcker, a former Federal Reserve chairman whose voice was being drowned out, got a meeting with the president. The result: tougher measures than the president’s top economic advisers were advocating.

And she is the president’s protector in chief, or as Treasury Secretary Timothy F. Geithner put it, the person who must be as “omniscient as possible” in spotting trouble on the way. Those whom she deems to have failed Mr. Obama tell of scolding late-night calls and her trademark accusation of betrayal: “You are hurting the president.”

But she has also steered him toward controversy, as in the contraception debate. And some of Mr. Obama’s most senior advisers worry — perhaps not entirely without jealousy — that her direct access to the president has at times led to half-baked decision making and unclear lines of authority.

Mr. Obama’s first two chiefs of staff, Rahm Emanuel and Mr. Daley, clashed with Ms. Jarrett over strategic direction and over who had greater authority to interpret and carry out the president’s wishes, several officials said.

“He’s got a real mess in the West Wing,” said one close presidential adviser. “Valerie is effectively the chief of staff, and he knows, but he doesn’t know. She’s almost like Nancy Reagan was with President Reagan, but more powerful.”