Romney also handily won Arizona, a primary state all but ignored by the other Republican presidential hopefuls.
The former Massachusetts governor's first stop after winning Michigan with 41 percent of the vote, compared with Santorum's 38 percent, was to be a rally in Toledo, Ohio, followed by a town hall in the Columbus suburb of Bexley, his campaign said.
Romney won by a far easier margin in Arizona, taking 48 percent of the vote to Santorum's 27 percent, with most precincts reporting. Gingrich had 16 percent and Paul had 8 percent.
The Arizona victory was Romney's easiest win of the season, giving him all 29 delegates. It was unclear early Wednesday how Michigan's 30 delegates would be divvied up.
Santorum, a former senator from Pennsylvania, planned to campaign in Knoxville, Tenn., Wednesday, followed by Ohio, his campaign said.
Ohio and Tennessee are two of the biggest Super Tuesday contests March 6 in terms of delegates, and recent polls suggest both have the potential to be winners for Santorum.
Super Tuesday involves 10 primaries and caucuses awarding a total of 437 delegates in Alaska, Georgia, Idaho, Massachusetts, North Dakota, Ohio, Oklahoma, Tennessee, Vermont and Virginia.
Speaking to supporters in the Detroit suburb of Novi Tuesday night, Romney said, "We didn't win by a lot, but we won by enough, and that's all that counts."
Santorum told supporters at a Grand Rapids, Mich., rally, "A month ago, they didn't know who we are -- but they do now."
Paul spoke Tuesday night from Virginia, a state where only he and Romney qualified for the ballot next week.
He pledged to stay in the race, declaring his campaign was "still winning a lot of delegates, and that's what counts."
Gingrich, who did not actively campaign in Michigan or Arizona, held a rally with some 500 supporters at the University of West Georgia in Carollton Tuesday night.
His allies were to begin airing a new "super PAC" TV ad on his behalf starting Wednesday, aggressively taking on Romney across several Southern states.