Newt Gingrich has come the closest yet to acknowledging what just about everyone else in the political world has already decided: that he has no chance of becoming the Republican presidential candidate and that Mitt Romney is going to be the winner.

Interviewed on WTMJ radio on the campaign trail in Wisconisn ahead of its Tuesday primary, Gingrich virtually admitted defeat, conceding that front-runner Mitt Romney will "probably" secure the necessary 1,144 delegate majority.

But he still insisted he is still not yet ready to wind up his own campaign.

Usually, a candidate in Gingrich's positition would quit: he has won just two states, with little chance of any further victories, his funds have dried up, he has cut his campaign staff and the last two reporters following him full-time have been withdrawn.

In spite of the humiliation, he insists he will remain in the race until the Republican convention in Tampa, Florida, in August or until such time as Romney reaches the magic 1,144 majority of delegates, which could happen in May or June.

Gingrich earlier this month was much more bullish about his prospects and dismissive of Romney's status as frontrunner. He predicted that Romney would fail to secure the 1,144 delegates, that the decision would be brokered at the convention and that Romney was not much of a frontrunner, having done poorly in the Mississippi and Louisiana primaries.

But Gingrich, who had a private meeting with Romney in Louisiana last week, was noticeably less bombastic in his WTMJ interview on Friday. He said that Romney is "clearly the frontrunner" and "will probably" get the 1,144 majority, though adding the caveat that he would have to earn it.

The more positive approach to Romney is a marked departure from the Florida primary in January, the low point in relations between the two, when Gingrich expressed bitterness over a relentless media blitz.

Polls show Romney is favourite to win not just Wisconsin on Tuesday but also Washington, DC and Maryland. An NBC/Marist poll published Friday showed Gingrich not only trailing Romney and Rick Santorum in Wisconsin but in fourth place behind Ron Paul. It showed Romney on 40%, Santorum on 33%, Paul on 11% and Gingrich 7%.

Gingrich arrived in Wisconsin on Thursday to campaign throughout Friday and Saturday. Television crews are still following him but the last two print journalists permanently attached to him, from the Associated Press and the Atlanta Journal-Constitution, from his home state Georgia, were withdrawn last Friday.

Jay Bookman, a columnist on the Atlanta Constitution-Journal, wrote on Thursday: "He has no money and in fact his campaign is deeply in debt. He lacks the resources to travel, let alone buy campaign ads, and the press has abandoned him, raising the age-old question: if a man utters pearls of profound wisdom, and no-one is there to hear him, did he really utter them at all? Yet still he presses on, insistent against all evidence that he remains relevant." Fox News labels Gingrich's bid as "the Zombie campaign".

The biggest blow to Gingrich has been the loss of funding by Sheldon Adelson, the Las Vegas hotelier and casino owner, whose family provided $16.5m of the $18.5m that went into Gingrich's Super Pac. Adelson, in video footage released this week, said Gingrich may be "at the end of his line".

Gingrich can expect to face questions about quitting when he appears Sunday on CBS's Face the Nation along with Paul.

Romney picked up another major endorsement on Friday when the author of the Republican budget passed by the House Thursday, congressman Paul Ryan, backed him. Ryan is popular with the Tea Party and is widely seen in the Republican party as a future presidential candidate.

Ryan, who is to campaign on Romney's behalf in Wisconsin this weekend, called on Santorum, Gingrich and Paul to quit. He told the Milwaukee Journal Sentinel: "I believe it's getting to the point where it's going to become counter-productive if the primary drags on. It's going to get much tougher to defeat Barack Obama in the fall."