Mitt Romney has won all three US Republican Party primary contests in Wisconsin, Washington DC and Maryland.
The contests come at a crucial time in the primary season, with Mr Romney holding a commanding lead over his rivals.
Many senior Republican party officials have also been saying it is time for the other contestants to bow out of what they say is an increasingly damaging competition.
Mr Romney went into the contests hoping he could finish off at least some of his opponents.
He has a significant lead in the delegates needed to win the nomination and he is in a very strong, some would say unassailable, position.
However, his closest challenger Rick Santorum is still gaining support among rural and deeply conservative voters and he has the contest in his home state of Pennsylvania still to come.
Newt Gingrich has also stayed in the race, even though his campaign has faltered.
Ron Paul has vowed to continue, even though he has failed to win a single state.
Senior people within the Republican party are anxious that the challengers should bow out and allow Mr Romney to go on to challenge Barack Obama in November, but none of them seem inclined to call it a day just yet.
"Thank you to Wisconsin, Maryland and Washington, DC. We won them all! This really has been quite a night," Mr Romney told supporters in Milwaukee, Wisconsin.
"We've won a great victory tonight in our campaign to restore the promise of America."
But Mr Santorum has insisted the Republican race to see who will challenge President Obama in November was merely at "half time," and that he would carry on at least into May, when there will be a series of primaries in states more favourable to his candidacy.
"The clock starts tonight," Mr Santorum told supporters in his home state of Pennsylvania, which votes later in April.
"We've got three weeks to go out here in Pennsylvania and win this state. And after winning this state, the field looks a little different in May," he said.
Mr Romney is now more than half way to the magic number of 1,144 delegates needed to be crowned the Republican flag-bearer at the party's national convention in Tampa, Florida in August.
