Mitt Romney swept three Republican primaries in Indiana, North Carolina and West Virginia, moving closer to sealing the party nomination for November presidential polls against incumbent Barack Obama.
After his latest victories, the former Massachusetts governor has 934 of the 1,144 delegates needed to clinch the nomination, according to CBS News estimates.
Romney faced no serious opposition in Tuesday's primaries in Indiana, North Carolina and West Virginia.
He spent the day campaigning in Michigan, where he attacked Obama as an "old-school liberal" and claimed that the president's policies would take the country backward.
In the absence of serious competition, Romney was able to collect the bulk of the 100 delegates at stake in Tuesday's primaries.
The Republican party will officially nominate its candidate for the presidential elections in its convention in Tampa, Florida at the end of August.
Romney's former rivals Newt Gingrich and Rick Santorum have already dropped their bid for the presidency. Santorum endorsed Romney late Monday night in an email to supporters.
Now, Ron Paul, who has so far secured an estimated 92 delegates, remains in the Republican race.
During an interview to Fox News, Romney sought to portray himself as an underdog against the president, saying: "We're gonna have to fight very, very hard to break through the clutter of the charges and the attacks and the efforts to dissuade people from looking at his record."
Obama and his potential Republican rival are expected to witness a tight contest, according to latest opinion polls.
In a Gallup/USA Today poll of 12 swing states released this week, Obama edged out Romney 47 per cent to 45 per cent. The result is within the poll's four-point margin of error and closer than Gallup's last swing state poll from March, when Obama held a nine-point lead.