Mitt Romney has scored another victory over his chief rival Rick Santorum - this time in Puerto Rico's Republican presidential primary.
The victor is already campaigning in the critical heartland state of Illinois ahead of that state's primary on Tuesday - another chance to prove he is the best choice to challenge US president Barack Obama in the November election.
Romney, the former Massachusetts governor, has been unable to win the hearts of the party's conservative base, who distrust him for his moderate past positions on social issues like abortion and gay rights.
Rick Santorum, a former Pennsylvania senator, said he was in the contest for the long haul because Mr Romney is a weak front-runner, even though he comfortably leads in the fight for delegates to the nominating convention.
Mr Santorum was campaigning in the southern state of Louisiana, which holds its primary on Saturday.
"This is a primary process where somebody had a huge advantage, huge money advantage, huge advantage of establishment support and he hasn't been able to close the deal and even come close to closing the deal," Mr Santorum said of Mitt Romney. "That tells you that there's a real flaw there."
To gain the nomination, Mr Romney must accumulate 1,144 delegates to the Republican National Convention - allocated through state-by-state primary elections and caucuses. Romney is on pace to capture the nomination in June unless Santorum or Gingrich is able to win decisively in the coming contests.
Before Puerto Rico's vote was in, Mr Romney had 501 delegates, more than all of his rivals combined. Rick Santorum stands at 253, former House Speaker Newt Gingrich has 136, and Texas representative Ron Paul is at 50, according to an Associated Press projection.
With 61% of the Puerto Rican votes counted, Mr Romney had 83% of them. He won all 20 delegates to the national convention at stake because he prevailed with more than 50 percent of the vote. That padded his comfortable lead over Santorum in the race to amass the 1,144 delegates needed to clinch.
Both Santorum and Romney campaigned last week in Puerto Rico, the US commonwealth island in the Caribbean, where residents are US citizens but cannot vote in the November presidential election. Each day that passes without a certain Republican nominee and the extension of the bitter fight raging with conservative opponents Santorum and Gingrich has been seen to emphasise disunity in the party, creates divisions and plays into the hands of Barack Obama.