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 nominees from both the Republican and Democratic parties 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.
Key dates to come
1 March – “Super Tuesday” – 15 states or territories decide
15 March – Five big states vote, including Ohio and Florida
18-21 July – Republican convention, nominee picked
25-28 July – Democratic convention, nominee picked
8 November – US presidential elections
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.
Protesters, including some from the Black Lives Matter movement, repeatedly disrupted a Trump rally in Radford, Virginia, on Monday after he was criticised for failing to condemn a Ku Klux Klan white supremacist leader in an interview.
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.
Democrats are voting in Alabama, Arkansas, Georgia, Massachusetts, Oklahoma, Tennessee, Texas, Vermont, Virginia, Colorado and Minnesota, as well as in the US territory of American Samoa. Democrats abroad will also submit their votes.
Mrs Clinton is eyeing black voters in places like Alabama, Georgia and Virginia after taking eight out of 10 black votes in South Carolina.
Jane Sanders said her husband faced a “rough map” on Super Tuesday but would take his campaign through to July’s Democratic convention.
“We expect to win some states and lose some states tomorrow and we think it will only get better as it goes along,” she said on Monday.