WASHINGTON — Mitt Romney has all but won the 2012 Republican presidential nomination, top senator Lindsey Graham said Sunday, agreeing with the candidate that "mathematically, this thing is about over."

Romney has won 14 of 25 state-by-state votes that decide which Republican candidate takes on President Barack Obama in November, compared to eight wins for Rick Santorum and just two for former House speaker Newt Gingrich.

These victories have given Romney almost 40 percent of the 1,144 delegates needed to secure the nomination. He has 446 delegates, Santorum 199 and Gingrich 117, according to authoritative poll aggregator RealClearPolitics.

"Mathematically, Rick would have to win 75 percent of what remains," Graham, a senior Republican who serves on various Senate committees, told ABC's "This Week" program.

"He's done an outstanding job, Rick has, of starting with almost nothing and being a real contender, and Newt's come back from the dead two or three times," Graham said. "But mathematically, this thing is about over, but emotionally it's not."

Graham was speaking ahead of two do-or-die contests for Gingrich on Tuesday in the conservative southern states of Mississippi and Alabama -- although the former House speaker has pledged to stay in the race until the bitter end.

The Santorum camp argues that if he can consolidate the conservative vote behind him, at Gingrich's expense, then he can still overtake the frontrunner before the race wraps up at the party's end of August convention.

"I think everybody believes, if I could just get a one-on-one with Romney, I could win this thing," said Graham.

"But if Romney does well, wins either Mississippi or Alabama and wins Illinois, then I think it's virtually impossible for this thing to continue much beyond early May."

Graham, who has yet to officially endorse any candidate, stopped short of calling on Santorum, Gingrich, or even veteran Texas congressman Ron Paul -- who has yet to win even one state vote -- to quit.

"It's Romney's to lose," he said. "And, quite frankly, every time he had his back against the wall, he's performed. And I like his chances, but the other two candidates have got to make that decision themselves."