The 70 delegates at stake in Minnesota (37) and Colorado (33) were the biggest one-day total so far in the GOP race to name an opponent for President Obama.
Tuesday's caucuses were the first step of a multistep process to determine Minnesota's delegates. An Associated Press analysis showed Santorum would win all the statewide delegates if he maintains the same level of support throughout the process.
Missouri's delegates will be chosen beginning in caucuses expected to draw far more competition from Romney, Newt Gingrich, and Texas Rep. Ron Paul next month.
Romney won in Minnesota and Colorado in 2008, the first time he ran for the nomination, but the GOP has become more conservative in both states since then under the influence of tea party activists.
Santorum, a former Pennsylvania senator, campaigned aggressively in all three states, seeking a breakthrough to revitalize a campaign that has struggled since his narrow first-place finish in the Iowa caucuses a month ago.
In Missouri on Tuesday night, Santorum said his wins were "a victory for the voices of our party." Romney, he said, "would not be the best person to . . . fight for the voices of freedom in America."
And to Obama, he said, "You'd better start listening to the voice of the people."
"Why would you think he would be listening now?" he said of the president. "He thinks he's smarter than you."
Addressing supporters late Tuesday in Denver, Romney offered congratulations to Santorum but said, "We'll keep campaigning down the road, but I expect to become the nominee with your help."
"When this primary season is over, we're going to stand united as a party behind our nominee to defeat Barack Obama," Romney said.
