Donald Trump is leading the Republican polls in at least eight of the 11 Super Tuesday states
Millions of voters are heading to the polls for Super Tuesday as front runners Hillary Clinton and Donald Trump aim to take unassailable leads over their rivals for the White House.
Democrats are voting in 11 states as the candidates continue their quest to win the 2,383 delegates required for the Democratic nomination.
Mrs Clinton (with 546 delegates) currently leads Bernie Sanders (with 87 delegates), with 859 delegates at stake on Super Tuesday.
Republicans are also casting their votes in 11 states as the candidates aim for the 1,237 delegates to win the nomination at the July convention.
Billionaire Mr Trump (82 delegates ) was ahead of Texas Senator Ted Cruz (17 delegates) and Florida Senator Marco Rubio (16 delegates) heading into a day when 595 delegates are up for grabs.
The first polls opened in Virginia at 6am (11am Irish time) and most will close between 7pm and 8pm (midnight and 1am Irish time).
Vermont Senator Mr Sanders was up early to vote in his home town of Burlington, telling reporters that if turnout was high "we are going to do well; if not, we're probably going to be struggling".
Mr Sanders was thrashed by Mrs Clinton in South Carolina at the weekend, with the latter claiming 86% of the African-American vote.
A new CNN/ORC poll suggests that either Democratic candidate would easily defeat Mr Trump if the presidential election was held now rather than in November - but if Mr Rubio or Mr Cruz was the Republican candidate, it would be a much closer race.
Mr Trump is leading the polls in at least eight of the 11 Super Tuesday states, with CNN/ORC putting him on 49% nationally.
Mr Rubio is second (16%) followed by Mr Cruz (15%), retired neurosurgeon Ben Carson (10%) and Ohio Governor John Kasich (6%).
Texas is a big prize on Super Tuesday, with Mr Cruz banking on winning his home state.
Meanwhile, Mr Trump saw a rally in Virginia on Monday repeatedly disrupted by demonstrators.
He ordered several to be removed and asked one, who interrupted his remarks on immigration: "Are you from Mexico?"
Speaking in Fairfax, Virginia, Mrs Clinton attacked the Republican campaign rhetoric.
"Scapegoating people, finger-pointing, blaming," she said. "That is not how we should behave toward one another."
States holding contests in both parties on Tuesday are Alabama, Arkansas, Georgia, Massachusetts, Minnesota, Oklahoma, Tennessee, Texas, Vermont and Virginia. Republicans vote in Alaska and Democrats in Colorado.