The former Massachusett governor stormed to a hat-trick of three primary wins on Tuesday night, defeating main rival Rick Santorum in Maryland by winning nearly 50 percent of the vote.
Santorum has rejected calls to abandon his candidacy in the name of party unity, defiantly vowing to fight on.
With an extensive lead in the race to win the Republican candidacy for the November presidential election, Romney has turned his attention toward US President Obama.
On Monday, Romney attacked Obama’s administration for failing to pass the DREAM Act or comprehensive immigration reform during the first two years of his administration, when he had majorities in both houses of Congress.
The Obama campaign responded by characterizing Romney as an immigration hardliner, saying that the Democrats failed to pass reform because of Republican opposition.
Republican nomination will be officially awarded at the party's convention in August. The eventual winner will challenge President Barack Obama for the White House on November 6.
The US president is not elected directly by people. The Electoral College does the voting, which means a candidate could become president despite losing the popular vote.