Senators had been debating a contentious immigration overhaul for hours when Obama — the Illinois Democrat running for president — introduced an amendment that would end a key provision after five years rather than 14.

His target was a proposed point system that would make it easier for would-be immigrants to obtain visas based on their education levels or work skills rather than on having close relatives already living in the United States. "We can't weaken the essence of what America is by turning our backs on immigrants who want to reunite with their family members," Obama said in his speech.

The amendment infuriated Graham, a South Carolina Republican with close ties to another presidential hopeful, Sen. John McCain, R-Ariz. Pacing the Senate floor and waving Obama's amendment, Graham loudly accused Obama of undermining a delicate agreement whose advocates have shown political courage.

Issues that require bipartisanship often fail, Graham said, "because some people, when it comes to the tough decisions, back away." Obama's amendment, he said, would destroy the bill's prospects and bring special woe to Republicans — such as himself — who have endured conservatives' searing criticism for backing it.

It would undercut "everybody over here who's walked the plank and told our base, 'You're wrong,"' Graham said. "So when you're out on the campaign trail, my friend, tell them about why we can't come together. This is why."

Obama briefly appeared stunned and demanded time to respond. The notion that his amendment would gut the bill "is simply disingenuous" he said. "It's engaging in the sort of histrionics that is entirely inappropriate for this debate."

Almost immediately, the two men continued the argument in a hall just outside the chamber. "They were going at it," said Sen. Mel Martinez, R-Fla. "We could hear them inside."

In separate interviews later, Graham said, "I wanted to go outside" to impress upon Obama the danger he was causing for a bill he supposedly supported. "I said, 'I'm very disappointed in you,"' he said, adding: "I like the fellow."

Obama said Graham was overstating the potential impact of "a mild amendment."

"It's a matter of too much coffee and people being on the floor too long," he said.