Republican strategists have long said that the party's search for a presidential candidate meant a choice between its ideological heart and its election-winning head.

As the first of the Republican primaries drew closer, there were growing signs that voters had sobered up from their indulgence of the Tea Party movement and were ready to plump for a candidate with a real shot at beating Barack Obama next year. That man was Mitt Romney.

Now it appears the heart might win out after all, in the unlikely form of Newt Gingrich. A career politician and former speaker of the House of Representatives, his sudden rise is likely to be strengthened by Herman Cain's decision this weekend to drop out of the race following revelations about his private life.

In the past months, other Republican candidates, including the Texas governor, Rick Perry, have risen to challenge Romney and then been dramatically brought down by public exposure of their inadequacies. But none took as strong a lead as Gingrich now commands on an anti-Washington, and fiscal and social conservative stance.

However, Gingrich is vulnerable to what rivals describe as his "serial hypocrisy" – for claiming to be a political outsider after decades in Washington, for being a paid lobbyist for corporations taking big government subsidies, and for his long history of adultery.

He drew stinging criticism recently for saying labour laws should be relaxed to let children work as school janitors.

Gingrich was confident that he is now the favoured candidate for the Republican nomination even before Cain dropped out.

"It's very hard not to look at the recent polls and think that the odds are very high I'm going to be the nominee." he told ABC News last week.

Opinion polls consistently show that Romney, the former governor of Massachusetts, stands a far better chance than Gingrich of beating Obama. Most surveys show Obama ahead of Romney by less than 2%. Against Gingrich, the president leads by up to 12%.

But the polls also show that many Republican voters are distrustful of what they regard as Romney's history of liberal positions on health care insurance and abortion rights, and his flip-flopping on those and other issues. Across the country, polls put Gingrich ahead of Romney by up to 20% among likely Republican voters.

One of the most recent polls shows Gingrich pulling away from the pack in Iowa, where the first selection of the Republican candidate takes place in caucuses next month. The Des Moines Register survey, taken before Cain quit, gives Gingrich 25% support among Republican voters. Romney, at 16%, is pushed into third place, 2 points behind Ron Paul.

Gingrich's support has also picked up dramatically in New Hampshire, the first state to have a full primary vote in mid-January, according to an NBC poll released on Sunday. Although he is in second place there, Gingrich has risen from just 4% to 23% support since October. Romney is still in the lead, with 39%. But his support is sliding dramatically. Cain was commanding around 10% support among Republicans before he dropped out on Saturday. Republican strategists expect the bulk of that support to go to Gingrich, who went out of his way to praise Cain within hours of his campaign being suspended.

But as frontrunner, Gingrich will come under closer scrutiny. He is vulnerable to accusations of being a Washington insider after his many years in Congress and then taking money as a political consultant for big business, and over his admission of serial adultery in two of his three marriages. A conservative Republican senator, Tom Coburn, said he will not back Gingrich after criticising "leaders that have one standard for the people they are leading and a different standard for themselves".

Paul called Gingrich a "serial hypocrite" in his latest campaign advertisement for being on both sides of issues such as global warming, and for taking money from the health care industry to back compulsory insurance while publicly opposing it.

Fellow Republican candidate Michele Bachmann recently attacked Gingrich as "memory challenged" over his denial of his previous support for amnesty for illegal immigrants.

But Romney is as vulnerable, if not more so, over changing his position. Last week he grew irritated in a Fox News interview at questions over his shifts on abortion rights, health care and other issues close to the hearts of conservatives.