If Democrat Elizabeth Colbert Busch eeks out a win Tuesday against former South Carolina Gov. Mark Sanford in the 1st District special election, she would immediately become the nation's most vulnerable Democrat. The reason: Her district is heavily Republican.

"Colbert Busch will have to hang on for dear life," said Kyle Kondik, the House race analyst for the University of Virginia's Center for Politics.

The race to fill the seat opened when Gov. Nikki Haley appointed Rep. Tim Scott to the Senate. The contest is receiving massive media attention, mostly because Colbert Busch is the sister of TV comedian Stephen Colbert and Sanford is the star of a marital scandal.

The race is rated a tossup by most analysts, including the Democratic Congressional Campaign Committee, and Sanford has a tiny edge in polls.

But the political reality of the district shows how hard it will be for Colbert Busch to keep the seat. Consider that Mitt Romney beat President Obama in the district 58 percent to 40 percent. There are only three other Democrat-held House seats in districts where Romney fared better.

And in reviewing 45 special House elections over the past decade, Kondik said only four candidates won in congressional districts where the other party's presidential candidate did better than Romney's 18-point margin, and none of those winners is still in Congress.

Kondik on mid-Monday tweeted his expectations that Sanford would in in a come-from-behind race. "Despite indications that SC-1 is a legit toss-up, I'm willing to make a highly tenuous pick - I favor Sanford b/c of the district," said his tweet.