The Republican party was reeling on Wednesday from Rick Santorum's stunning hat-trick of victories that overnight changed the landscape of the presidential nomination and raised doubts about the viability of frontrunner Mitt Romney.

Santorum's wins in Colorado, Minnesota and Missouri, which Romney had been expected to take, means the Republicans are facing a long, brutal and costly nominating campaign.

Until Tuesday night, Romney, with victories in Florida and Nevada chalked up, had been cruising towards the nomination, widely predicted to emerge as winner of the February contests. But Santorum up-ended those predictions and turned it into a three-man race for the nomination to take on Barack Obama for the White House in November.

Santorum said that donations are now flowing into his previously cash-strapped campaign as a result of his victories.

Santorum, who fought the caucuses and primaries on a relatively small budget compared with the other three candidates in the race, said the past fortnight had been the best so far for fundraising, as conservatives realised he was a viable alternative to Romney and Gingrich and that he had the momentum going into the next round of elections.

As the results came in on Tuesday night, Santorum said he received a massive injection of donations online – about $250,000. Wyoming businessman Foster Friess, who backs Christian causes and funded Santorum through the opening contest in Iowa, is also prepared to provide more cash.

"We are going to have money to make the case," Santorum said.

After its disastrous showing on Tuesday, the Romney campaign hit back hard at Santorum on Wednesday in a foretaste of its strategy to counter the new threat. He had hoped to spend February consolidating his frontrunner status and to focus his attacks on Obama. Instead, he will have to concentrate on his rivals and Santorum is likely to face the kind of multi-million negative ad campaigning that proved so effective for Romney against Gingrich in earlier states.

Andrea Saul, Romney's spokeswoman, portrayed Santorum and Gingrich as Washington insiders, a term of abuse for conservatives. "Speaker Gingrich and senator Santorum have over half a century's worth of time in Washington between them. They can't fix our country's spending problem because they helped create it," Saul said.

She added that Santorum "renounced his belief that deficits are bad and voted to raise the debt ceiling by trillions – all while supporting billions in pork-barrel spending in Pennsylvania and across the country. That is not a record that fiscal conservatives will embrace once they know the facts."

Interviewed on CNN, Santorum replied that Romney was more of an insider than he was, and described the former Massachusetts governor as unprincipled. Recalling his own defeat in a Senate race in Pennsylvania in 2006, Santorum said: "A lot of folks lose races. What I didn't lose, unlike governor Romney, was my principles."

Larry Sabato, professor of politics at the University of Virginia, said Santorum's victories leave the Republican party in "a mess". Sabato said he did not anticipate Santorum becoming the nominee and if it were to happen, Santorum would lose.

Sabato said he saw Santorum's clean sweep as "the conservative base lashing out at Romney. 'We do not like you. We do not trust you'."

He added: "This race is probably going to go the distance, and Romney did not want this."

Santorum won Colorado with 40% of the vote to Romney's 35%, 13% for Gingrich and 12% for Texas congressman Ron Paul. In Minnesota, Santorum took 45% to 27% for Paul, with Romney coming in a humiliating third on 17% and Gingrich on 11%.

Colorado has 36 delegates at stake and Minnesota 40. Missouri had no delegates up for grabs: its convention to choose delegates is not until 17 March, and it only decided to hold its primary to try to seize a share of media attention.

The potential turning point could be Super Tuesday on 6 March, when 10 states hold elections. Victories for Gingrich and Santorum in some of these states, and a share of delegates in others, would doom Romney's chances of bringing the race to a close in April.

Following last night's contests, Romney has only 107 delegates, well short of the 1,144 he needs to win the nomination. Santorum has 45 delegates, Gingrich 32 and Paul nine.