With the primary election season in full gear, it's shaping up to be a highly competitive match between the Democratic candidates former Secretary of State Hillary Clinton and Sen. Bernie Sanders, I-Vt., for the party's nomination. The tight race means there's a lot at stake when the two go head-to-head Thursday night at the Democratic debate as neither can afford even a slight dip in the polls. 

The primary season kicked off Feb. 1 with what the Iowa Democratic Party called a "historically close" vote at the state's caucuses. Results showed Clinton only narrowly ahead of Sanders, winning 49.9 percent of the vote to Sanders' 49.6 percent. Even though Clinton remains ahead in national polls, her resounding defeat in New Hampshire Tuesday and the Iowa caucus indicated her campaign is vulnerable to Sanders' rise.

The next major battlefront is the first Southern primary of the presidential race: South Carolina. Clinton is comfortably ahead in the latest NBC News-Wall Street Journal poll, which reported she has 64 percent support among Democratic primary voters while Sanders has 27 percent.

Nationally, the race is much tighter. Quinnipiac University released a poll Friday that showed Clinton with 44 percent of support from Democratic voters to Sanders’ 42 percent. The Vermont senator is edging up on the Democratic front-runner. In December, Sanders received 30 percent support in the Quinnipiac University poll while Clinton led with 62 percent. “Democrats nationwide are feeling the Bern as Sen. Bernie Sanders closes a 31-point gap to tie Secretary Hillary Clinton,” said Tim Malloy, assistant director of the poll.

Clinton and Sanders will square off for the last time Thursday before the South Carolina primaries. PBS will host the debate, which is scheduled to air at 9 p.m. EST.