Mitt Romney will win all three of Tuesday's Republican presidential primaries, including Wisconsin, CNN has projected.

The hat-trick means Romney has taken a stride closer to the Republican presidential nomination.

His main rival Rick Santorum, who has faced calls to bow out of the race in the name of party unity, defiantly vowed to fight on.

The eventual winner will face Barack Obama in November's election.

Romney's wins in Wisconsin, Maryland and the District of Columbia will put him past the halfway mark to the 1,144 delegates needed to clinch the nomination and add to a wide delegate lead that he holds over the other major GOP presidential candidates, according to CNN estimates.

Yet Santorum indicated Tuesday night he would fight on to the April 24 primaries in five states, including his home state of Pennsylvania, where his campaign hopes a win would be a gateway to a run of May primaries in states where he can capitalize on higher percentages of conservative voters.

The former Massachusetts governor is expected to take the majority of Wisconsin's 42 delegates and Maryland's 37 delegates, with some designated solely for the winner and the rest to be awarded proportionally.

He will take all 16 delegates at stake in the District of Columbia.

Former House Speaker Newt Gingrich and Texas Congressman Ron Paul trailed far behind in the primary results.

Exit polls have indicated that Republican voters were primarily concerned with a candidate's ability to defeat Obama.