A Fox News poll conducted from Monday through Thursday last week shows Mitt Romney losing support while Rick Santorum surged into a virtual tie with the former Massachusetts governor.

In interviews conducted on Monday and Tuesday nights -- immediately before the news of his victories -- Santorum received the backing of 17 percent of GOP primary voters. That was well behind Romney (35 percent) and Newt Gingrich (26 percent), and slightly ahead of Ron Paul (14 percent).

In interviews conducted on Wednesday and Thursday nights -- after his wins -- Santorum's support nearly doubled, which put him tied at the top with Romney for those two days at 30 percent. That's an increase of 13 percentage points. Over the last two nights, Romney also received 30 percent, a drop of 5 points. Gingrich came in at 16 percent, down 10 points. Paul's support held steady at 15 percent.

Looking at the results from all four nights of this week's interviewing, Romney retains his frontrunner spot with 33 percent, followed by Santorum at 23 percent, Gingrich at 22 percent and Paul at 15 percent.

The previous Fox News poll was conducted in mid-January, and since then Rick Perry and Jon Huntsman dropped out of the race. Romney's support is down 7 percentage points since the January poll, while Santorum is up 8 points, Gingrich is up 8 points and Paul is up 2 points.

For the four nights of polling, voters who are part of the Tea Party movement back Gingrich (34 percent) over Santorum (28 percent) and Romney (25 percent). Paul receives the support of 9 percent of Tea Partiers. White evangelical Christians break for Santorum (31 percent) over Romney (24 percent) and Gingrich (23 percent).

The Wednesday-Thursday totals have a higher margin of error because fewer people were interviewed. But Santorum's surge was so pronounced that there is no doubt it is real, and building.

Santorum was no doubt helped by Obama's blunder with Catholics last week. It has brought social issues front and center in the campaign which can only help the former Pennsylvania senator solidify his hold on the party's base.

Romney will give his CPAC speech today as will Santorum. It should be interesting to compare and contrast the two addresses. Both men have different goals in speaking at the conservative confab. Romney must seek to quiet fears that he would betray conservative principles once in office, while Santorum must convince those outside of the base that he is electable.