Mitt Romney won Tuesday's Republican presidential primary in Maryland, US media projected, as he hoped for a clean sweep in three contests aiming to consolidate his frontrunner status.

CNN, MSNBC and Fox television channels all called the vote for Romney just as the Maryland polls closed. Results from the US capital Washington and Wisconsin were not yet known as Romney bids for his party's nod to take on President Barack Obama in November elections.

Exit polls by CNN handed the former Massachusetts governor a large victory among both rural and urban voters in Maryland, which neighbors the capital.

With six percent of the vote counted, Romney was said to be on 53 percent over his main rival devout Christian former senator Rick Santorum on 27 percent.

Trailing behind were former House speaker Newt Gingrich on 10 percent and libertarian Texas congressman Ron Paul on eight percent. (AFP)