Republican candidates Donald Trump and Ted Cruz have emerged from the latest round of state voting as the two frontrunners for the nomiation, leading Trump to call for a "one-on-one" battle with Cruz and urging other rivals to bow out of the race.
Trump narrowly beat Cruz in Louisiana and Kentucky, while Cruz convincingly defeated Trump in Kansas and Maine.
Florida senator Marco Rubio came third in three states and last in Maine, where he didn't garner a single delegate.
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Speaking after the results, Trump said: "I would love to take on Ted Cruz one-on-one. Marco Rubio had a very very bad night and personally I call for him to drop out of the race. I think it's time now that he dropped out of the race. I really think so."
And Cruz, hinting at the same run off, said that "as long as the field remains divided, it gives Donald an advantage".
Trump's latest victories takes his tally to 12 states, but comes against a backdrop of growing concern about his meteoric rise from the Republican establishment, who largely wish to see him falter.
Bookmaker William Hill has cut the odds on Cruz becoming the Republican nominee from 8/1 to 3/1 following the results.
Meanwhile, in the Democrat side of the race, Bernie Sanders won Kansas and Nebraska, while Clinton took Louisiana.
Clinton won heavily in Louisiana, where she continued to win the support of African-American voters, which have not been so forthcoming with Sanders' campaign.