(CNN) -- Suddenly facing new questions about the strength of his candidacy, GOP presidential frontrunner Mitt Romney indicated Wednesday he's prepared to wage a more aggressive campaign against Rick Santorum, the surprise winner of all three of Tuesday's Republican contests.
"We think we can beat Sen. Santorum where we compete head-to-head in an aggressive way, and we obviously didn't do that in Colorado or Minnesota to the extent that (Santorum's) campaign did," Romney told reporters.
"There's no such thing as coronations in presidential politics. It's meant to be a long process," he added. "It's not easy to get the nomination (and) it's not easy to be elected president. ... And so far we're doing pretty well."
Romney also blasted Santorum and former House Speaker Newt Gingrich, calling them big-spending Washington insiders.
Republicans in Washington have "spent too much, borrowed too much, (and) earmarked too much," he said. "Frankly, Rick Santorum and Newt Gingrich were a big part."
On Tuesday, Santorum won caucuses in Minnesota and Colorado, as well as a nonbinding primary in Missouri. The wins -- Santorum's first since the Iowa caucuses in early January -- energized his campaign and signaled the Republican nomination fight will continue through Super Tuesday on March 6, when more than 400 delegates will be at stake.
"We definitely are the campaign right now with the momentum" and the "enthusiasm on the ground," Santorum said in a CNN interview Wednesday morning.
His campaign raised about a quarter of a million dollars online Tuesday night, Santorum said, adding that the last two weeks have been his best for fundraising.
A Santorum campaign aide said later Wednesday that the organization's website crashed during the day due to heavy volume.
The victories by the former Pennsylvania senator bolstered his contention that he is the strongest conservative challenger to the more moderate Romney for the GOP nomination.
"I don't stand here and claim to be the conservative alternative to Mitt Romney," Santorum declared Tuesday night to cheering supporters outside St. Louis. "I stand here to be the conservative alternative to Barack Obama."
Colorado was the most competitive state on Tuesday, with Santorum winning 40% of the vote to 35% for Romney, 13% for Gingrich and 12% for Texas Rep. Ron Paul
In Minnesota, Santorum got 45% of the vote to 27% for Paul, 17% for Romney and 11% for Gingrich, according to the secretary of state.
In Missouri, Santorum had 55% to Romney's 25% and 12% for Paul, according to unofficial results. Gingrich didn't make the ballot there.
Santorum's victories in two states Romney won in his unsuccessful 2008 presidential bid -- Colorado and Minnesota -- signaled both his growing support and Romney's inability to build on the momentum from victories in the two previous contests in Florida and Nevada.
However, a low turnout in all three races Tuesday signaled possible dissatisfaction among Republican voters with all of the candidates.
All the 70 delegates available Tuesday came from the Minnesota and Colorado caucuses, while the Missouri primary was nonbinding with no delegates at stake.
The two caucus states didn't officially award delegates Tuesday night -- that will happen down the road at district and state conventions -- but the news media, including CNN, used their results to make unofficial delegate count estimates. Romney ended the night with 115 delegates, compared to 38 for Gingrich, 34 for Santorum, and 20 for Paul, according to CNN's count.
To win the GOP nomination, 1,144 delegates are needed.
While Tuesday was a stunningly successful night for Santorum, it was a terrible night for Romney and for Gingrich, who has been competing with Santorum for the support of conservatives.
Gingrich spent little time or money in the three states, instead focusing his now-limping campaign on the Super Tuesday contests.
His campaign announced Wednesday that Gingrich will campaign next week in Georgia, the state he represented when he served in Congress and a neighbor of South Carolina, the only state Gingrich has won so far.
A senior adviser to Romney, meanwhile, signaled the campaign would take a tougher approach toward Santorum and portray him as a Washington insider.
"Look, I just don't think it's a time when people are looking to Washington to solve problems with Washington," senior Romney adviser Stuart Stevens said of Santorum.
Stevens downplayed Tuesday's results, saying: "We'd like to win everywhere, but you can't. And we've focused on key states for how we see a path to the nomination."
But Santorum struck back in his CNN interview Wednesday, describing Romney as a supporter of "big government" who didn't spent time in Congress only because he lost a race for the Senate in the 1990s.
Citing Romney's business experience, Santorum argued, "We're not running for CEO of the country. We're running for someone who can lead this country."
To Rep. Debbie Wasserman Schultz, the Democratic National Committee chair from Florida, Tuesday's results showed an underlying weakness in Romney's candidacy.
"What should have been a night where he began to consolidate Republican support instead has shown that Republicans are reluctant to get behind him," Wasserman Schultz said in a statement. "Republicans are giving the field of candidates another look, demonstrating that the more people get to know Mitt Romney, the less they like him. They know he'll say anything to get elected, and they don't want a candidate they can't trust."
She also cited "the lack of enthusiasm and low turnout we're seeing in these contests," adding that "no candidate embodies that dissatisfaction more than Mitt Romney."
For his part, Paul stressed his strength in the upcoming Maine caucuses on Saturday.
"Nobody else is about to at this point jump ahead of Romney," Paul told CNN. "But we think we're going to keep (going). We have a very good chance on what's happening up in Maine."
CNN's Josh Levs and Dana Bash contributed to this report.