Less than a week before both states' Republican primaries, the NBC Marist poll shows the former Massachusetts governor with a 43 to 27-percent lead in Arizona over the former Pennsylvania senator.
Former House Speaker Newt Gingrich has 16-percent support and Texas Congressman Ron Paul has eleven-percent.
In Romney's native state of Michigan he has the support of 37-percent of likely GOP voters while Santorum gets 35-percent, the two-point difference being well within the poll's margin of error.
Paul has 13-percent support in Michigan and Gingrich has eight-percent.
The four candidates will debate in Mesa, Arizona at eight Eastern time tonight on CNN.