The tension between the two Democratic presidential hopefuls, which has spilled into public view in the last three weeks, has been intensifying since January. It is clear that the genteel decorum of the Senate has given way to the go-for-the-jugular instinct of the campaign trail.
As the Senate held late sessions of back-to-back votes before its summer break, the two rivals kept a careful eye on each other as they moved across the Senate floor. For more than two hours one night, often while standing only a few feet apart, Mrs. Clinton and Mr. Obama never approached each other or exchanged so much as a pleasantry.
The scene repeated itself the next evening, a departure from the clubby confines of the Senate, where even the fiercest adversaries are apt to engage in the legislative equivalent of cocktail party chitchat.
When the cameras are on them, they can make a point of showing good sportsmanship. At a Democratic forum Saturday in Chicago, Mrs. Clinton smiled and moved her hands as though she was conducting a choir when an audience of liberal bloggers sang “Happy Birthday” to Mr. Obama, who was turning 46.
By the end of the event, Mr. Obama had called her “Hillary” in a sharp tone, criticizing her for accepting contributions from lobbyists.
The Clinton-Obama watch has become something of a parlor game, for their colleagues in Congress as well as for the scribes in the gallery above the Senate floor.
Consider a scene from the Capitol on a recent evening. It was a few minutes after 8 p.m. when the side doors of the Senate swung open and three Democratic candidates walked through.
Mrs. Clinton, of New York, and Senator Christopher J. Dodd of Connecticut came first, laughing as they made their way to the Democratic side of the aisle. A few paces behind was Senator Joseph R. Biden Jr. of Delaware, who quickly joined them. Mr. Obama, of Illinois, entered the Senate floor alone. He glanced at the other three, then pulled out his BlackBerry and paused for a few seconds before taking a seat next to three freshman senators. As the evening passed, Mr. Obama and Mrs. Clinton each spoke to several Republicans and to nearly every Democrat — except each other.
It was not always this way.
When Mr. Obama was running for the Senate in 2004, Mrs. Clinton once sat on the tarmac waiting out a lightning storm to fly to Chicago for a fund-raiser on his behalf. After he arrived in Washington in 2005, he studied her first year in office and worked to keep a similarly studious and low profile. After Hurricane Katrina, he joined Mrs. Clinton and former President Bill Clinton as they visited storm evacuees in Houston.
The relationship began to change 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, after the State of the Union address, the two senators found themselves doing back-to-back interviews 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.
Many Senate observers, even those close to Mrs. Clinton, say they believe she set the less-than-collegial tone. But Mr. Obama offered a glimpse into his own competitiveness two years ago when a Chicago television reporter told him about snagging a hallway interview with Mrs. Clinton.
“I outpoll her in Illinois,” Mr. Obama said. Then, realizing how his remark might sound, he added, “That was a joke!”
Now, with both candidates under Secret Service protection, their entourages are larger and they are less likely to have face-to-face encounters. One of the last times an impartial Senate observer could remember the two standing together without tension was when lawmakers gathered around a television in the cloakroom as Elizabeth Edwards, the wife of another Democratic presidential contender, John Edwards, announced that her cancer had returned.
In the public spotlight, they can be gracious toward each other. When asked at a debate last month in South Carolina what they liked and disliked about their opponents, Mrs. Clinton said of Mr. Obama, “I admire and like very much Barack, as I do with all of the candidates here.”
A moment later, Mr. Obama defended Mrs. Clinton against a bad fashion review Mr. Edwards had jokingly directed at her. “I actually like Hillary’s jacket,” Mr. Obama said.
As he walked through the Capitol recently, Mr. Obama paused for a moment to answer a question about their relationship.
“She’s said hello a couple times,” Mr. Obama said, a slow grin spreading over his face as he walked away.
Turning back, he added, “It’s been fine.”
Yet another reason why Clinton-Obama will never happen.