Cool Hand Hillary

When Mr. Obama was running for the Senate, Mrs. Clinton waited out a lightning storm on a tarmac to fly to Chicago for a fundraiser on his behalf. After he arrived in Washington in 2005, he studied her first year in office and worked to keep a similarly studious ? yet low ? profile. After Hurricane Katrina, he joined Mrs. Clinton and former President Bill Clinton as they visited storm evacuees in Houston, with Mr. Obama walking a few paces behind out of deference to the leading names of the Democratic Party.

The relationship began to change, according to several Democrats who are friendly to both senators, when Mr. Obama began musing aloud about a presidential bid. The day he opened his exploratory committee, several Senate observers said, he extended his hand and said hello on the Senate floor. She breezed by him, offering a cool stare.

One week later, following the State of the Union address, the two senators found themselves doing a back-to-back interview on CNN. Mr. Obama went first, with Mrs. Clinton pacing a few feet away. Finally, an aide escorted her completely around the rotunda of the Russell Senate Office Building, avoiding walking directly by Mr. Obama.