WASHINGTON  - Republican primary polls opened Tuesday in Maryland and the US capital Washington, with Mitt Romney seeking to extend his lead in the race for the nomination to face US President Barack Obama in November. Polls opened in Washington and the adjacent state of Maryland at 7:00 am, according to local officials, and were to open in Wisconsin - expected to be the most closely fought contest - an hour later. Romney is the favourite to win all three and further cement his lead, following key endorsements from senior party figures including, most recently, top Senate Republican Mitch McConnell.

His main rival Rick Santorum, a Christian conservative and former Pennsylvania governor, has insisted he has no intention of quitting the race, however, and former House speaker Newt Gingrich has also vowed to stay in.

Romney has won 21 out of 34 contests so far and amassed some 565 delegates, and after Tuesday he will likely have more than half the 1,144 delegates needed for the nomination.

Santorum has racked up 11 victories but has less than half Romney’s delegate count.

The nomination will be officially awarded at the party’s convention in August, with the winner going on to face Democratic incumbent Obama in the general election on November 6.