Mitt Romney easily won the Republican presidential primary in Illinois, increasing his already-formidable lead over chief rival Rick Santorum with another victory in a key industrial state.
Mr Romney, a multimillionaire who has the backing of most of the Republican establishment but struggles with the party's more conservative wing, is leading the overall race for delegates to choose a candidate to challenge President Barack Obama in the November election.
A less convincing win in Illinois would have triggered fresh anxiety within the party about his ability to win over its base.
Mr Santorum, a Catholic who is favoured by religious conservatives, has almost no hope of overtaking Mr Romney in the tally of delegates who will choose the nominee.
Returns from 95% of Illinois' precincts showed Mr Romney gaining 47% of the vote compared to 35% for Mr Santorum, 9% for Texas Representative Ron Paul and 8% for a fading former House Speaker Newt Gingrich.
Exit polling in Illinois showed Mr Romney was preferred by voters seeking a candidate to oust Mr Obama, and by those worried about the economy and federal deficit.
"I'm running for president because I have the experience and the vision to get us out of this mess," Mr Romney told supporters gathered in Schaumburg, Illinois, shortly after the results were announced.
Mr Santorum had robust backing from people seeking a real conservative and a contender with a strong moral character, and from the most religiously driven voters. But Illinois has only modest proportions of such voters.
Mr Romney, a former governor of Massachusetts, won at least 38 delegates in Illinois, with 32 still to be determined. He was leading with 560 delegates, according to The Associated Press count.
Mr Santorum had 253, Mr Gingrich 135 and Mr Paul 50. Delegates are allocated through state-by-state primary elections and caucuses, and a candidate needs 1,144 delegates to secure the nomination.
Neither Mr Gingrich nor Mr Paul campaigned extensively in Illinois.
Still, Mr Romney's inability to shake off a seemingly weak rival like Mr Santorum, an ex-US senator from Pennsylvania with little national following until a few months ago, highlights the struggles of his candidacy, reflecting his inability to generate excitement among Republican voters who do not trust the authenticity of his conservative views.
It also means Mr Romney has to spend time and money fighting primary battles instead of turning his full attention to defeating Mr Obama.
In a speech late yesterday, Mr Santorum appeared to play on voters' suspicions of Mr Romney, saying that Americans want "someone who you can trust ... someone who has a track record of deep convictions".
In Illinois, Mr Romney held an advantage in the competition for delegates to the Republican National Convention in August thanks to his superior campaign organisation.
Mr Santorum was ineligible for 10 of the 54 delegates at stake yesterday because his campaign didn't file the necessary paperwork.