Two weeks ago, Rick Santorum rode a three-state sweep to a 12-point lead in Rasmussen’s last national survey of likely Republican-primary voters.  Two days after Mitt Romney won a two-state sweep, Rasmussen’s latest poll shows him with a 16-point lead over Santorum and a lot of momentum:

Former Massachusetts Governor Mitt Romney, coming off his primary wins in Arizona and Michigan, has jumped to a 16-point lead over Rick Santorum in the battle for the Republican presidential nomination.

The latest Rasmussen Reports national telephone survey of Likely Republican Primary Voters shows Romney with 40% support to 24% for the former U.S. senator from Pennsylvania. This is Romney’s biggest lead to date and the highest level of support any GOP candidate has earned in regular surveying of the race. Two weeks ago, it was Santorum 39%, Romney 27%.

Former House Speaker Newt Gingrich earns 16% support, closely followed by Texas Congressman Ron Paul at 12%. Two percent (2%) prefer some other candidate, and six percent (6%) are undecided. The new findings mark virtually no change in national support for Gingrich and Paul.

The internals are just about what one would expect.  There is, interestingly, no real gender gap; the 16-point margin occurs among men and women alike.  Santorum wins among “very conservative” voters, but only barely outside the margin of error now at six points (39/33).  Romney has a 15-point lead among Republicans, and an 18-point lead among independents. Romney now edges Santorum by four among Tea Party voters (34/30) and by 25 among non-supporters of the Tea Party.  Although Santorum still has a double-digit lead among evangelicals (38/28), Romney wins Protestants by 19 points and Catholics by 27 points.

The continuing presence of Newt Gingrich as an option doesn’t have much impact on this dynamic.  In a hypothetical two-man race, Romney beats Santorum by twelve with a majority at 50/38.  Santorum is still seen very favorably by likely voters at 63/32, but Romney’s favorables now exceed Santorum’s, 71/25.  Romney does even better with Republicans, 74/23, as does Santorum with a 66/28.

The momentum shift comes at the worst possible time for Santorum.  He’s claiming that a tie in delegate allocations in Michigan should give him some credit for a victory in the state, but so far voters seem to be shifting towards the popular-vote winner.  With only five days between now and Super Tuesday, Santorum will be facing stiff headwinds in his attempt to keep Romney from re-establishing his inevitability argument and pushing Republicans to get on the bandwagon to bring the contested primaries to an end.  In response, Santorum can point to the whipsaw volatility in the polling over the last few weeks to show that it’s not over until someone gets a majority, but in order to maintain that argument, Santorum has to win some primaries, and soon.