Front-runners Donald Trump and Hillary Clinton have suffered defeats in the latest round of voting for the US presidential nomination in five states.
In the Republican race, Texas Senator Ted Cruz beat billionaire Mr Trump in both the Kansas and Maine caucuses.
In the Democratic race, Bernie Sanders has reportedly beaten Mrs Clinton in Kansas and Nebraska caucuses.
Mr Trump and Mrs Clinton are expected to triumph in the Louisiana contests, with Mr Trump also ahead in Kentucky.
Overall, Mr Trump is ahead in the Republican race. Mrs Clinton, a former secretary of state, leads the Democratic contest.
Efforts by Republicans to slow Mr Trump's march have until now failed.
Last week, on Super Tuesday, Mr Trump won seven of the 11 contests.
Speaking in Idaho, which votes on 8 March, Mr Cruz said: "What we saw in Kansas is a manifestation of a real shift in momentum."
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On Friday, Ben Carson, who was at one point a Republican front-runner, dropped out of the race.
The election itself, on 8 November, will see America vote for a successor to Barack Obama, a Democratic president standing down after two terms in office which have seen the Republicans take control of both houses of Congress.
Election calendar: Next races
- 6 March: Maine caucuses: Democratic
- 8 March: Hawaii and Idaho caucuses: Republican; Idaho primary: Republican; Michigan and Mississippi primaries: Democratic and Republican
- 15 March: Florida, Illinois, Missouri, North Carolina, Ohio primaries: all Democratic and Republican