Candidates bidding for their party’s ticket in the November US presidential election face their biggest test yet in the so-called Super Tuesday primaries.
Twelve states cast votes for candidates from both the Republican and Democratic party nominations in a contest seen as make-or-break for the hopefuls.
Contests stretch from Massachusetts in the east to Alaska in the north-west.
After earlier votes in four states, Donald Trump leads the Republican field and Hillary Clinton the Democrats.
Senator Ted Cruz cannot afford to lose to Mr Trump in Texas, his home state, while a reverse for Mr Trump in Massachusetts, with its moderate voters, could break the property tycoon’s nationwide momentum.
Mrs Clinton is hoping to build on her weekend victory in South Carolina, where she polled heavily among African-Americans, to restore her political fortunes after a bruising defeat in New Hampshire to Bernie Sanders, her self-styled democratic socialist rival.
On 8 November, America is due to elect 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.