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 either the Republican or Democratic parties or both in a contest seen as make-or-break for the hopefuls.
Contests stretch from Vermont in the east to Texas and Georgia in the south.
After earlier votes in four states, Donald Trump leads the Republican field and Hillary Clinton the Democratic.
The first polls opened in Virginia at 06:00 local time (11:00 GMT).
Senator Ted Cruz cannot afford to lose to Mr Trump in Texas, Mr Cruz’s 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.
Opinion polls give Mr Trump a lead in almost all of the 11 states holding Republican contests on Tuesday: Alabama, Arkansas, Georgia, Massachusetts, Oklahoma, Tennessee, Texas, Vermont, Virginia, Alaska and Minnesota.
The colourful campaign of the billionaire, who won three of the four early voting states, has divided Republicans.
On the eve of the polls, Nebraska Senator Ben Sasse became the highest-ranked elected party member to come out and say he would not back him for president.
He said he was “frustrated and saddened” and would look for a third option if Mr Trump won the Republican nomination.
Marco Rubio, the third-placed Republican contender after Mr Trump and Mr Cruz, is hoping to stay competitive, gambling on a win in his home state of Florida on 15 March. BBC
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