CNN and NBC called the three races a victory for Romney when the polls were barely closed.
According to initial projections of voters in the District of Columbia, Romney went into the election with a comfortable advantage. Romney, who has more than twice as many delegates against his main rival, , expected to ratify his status as the principal Republican candidate in yesterday's election.
Santorum, the more conservative former senator from Pennsylvania, relies on maintaining the support of the ultraconservative GOP base. Santorum continues to hammer at Romney's record in supporting abortion rights, gay couples and health care reform.
Last night there were a total of 92 delegates at stake: 39 Wisconsin, 16 of the District of Columbia and 37 in Maryland. In the latter two cases, the primaries were closed, that is, only registered voters participated in the Republican Party.
In Wisconsin, still open, non-registered members can vote.
Polling stations opened in Maryland and the U.S. capital Washington at 07.00 local time (11H00 GMT) and one hour later in Wisconsin.
The Republican establishment began to close ranks behind the candidacy of former Massachusetts governor. Senator Mitch McConnell said yesterday that Romney would be the insurance candidate to face President Barack Obama in the Nov. 6 election, where the Democrat seeking re-election.
Despite the gap in votes between Romney and Santorum, supporters of the latter in a wealthy residential area of Maryland said yesterday that their candidate should remain in the race to keep issues such as rising debt, public spending and social issues on the agenda.
"It's not over," said Stephanie Froehlich told AFP after voting with her husband at a church in , where the turnout was moderate.
"Romney does not have the 1,144 (delegates) he needs, and until we reach that golden figure we'll stay in the race," said Froehlich, a mother of 45.
Kurt Osuch, a supporter of former Speaker of the House of Representatives, , said after being the first to vote when they opened the doors of the church that their candidate should not give up.
To become the opponent of President Barack Obama in the election on November 6, Romney must have the 1144 party delegates needed to win the nomination at the August convention in Tampa, Florida.
Until yesterday, Romney had 566 delegates, according to RealClearPolitics, against Santorum's 263, Newt Gingrich's 140, and Ron Paul trails with 67. Romney now has more than half the needed delegates.
Meanwhile, President Obama yesterday attacked the "recipe for decline" as he called Republican proposals for deficit reduction. At the annual dinner of the Association of News Editors the President called the Repulbican agenda the "antithesis" of American history where the "prosperity" was not achieved by "the success of a few rich" but with "a strong and growing middle class."