With three weeks to go until the primaries, the three Republicans vying for the underdog spot in the upcoming special election for Secretary of State John Kerry's former Senate seat will debate at WBZ-TV studios in Boston starting at 7 p.m. tonight.

Tonight's debate will be the first major confrontation with the Republicans in the spotlight. The last GOP debate, on March 27, was essentially an opening act for the anticipated sparring match between Democrats Stephen F. Lynch and Edward J. Markey.

While several polls show that Markey is heavily favored to defeat Lynch in the April 30 primary, a clear frontrunner in the GOP race has yet to emerge. A poll conducted by WBUR in mid-March revealed that Michael J. Sullivan, a former United States attorney, had a sizable lead over his opponents, State Rep. Daniel B. Winslow and former U.S. Navy SEAL Gabriel E. Gomez. A more recent poll, conducted and released by the Gomez campaign, showed Gomez with a small lead over Sullivan and an advantage over Winslow.

What all the polls have in common so far, though, is that a sizable plurality of respondents have yet to decide. In the WBUR survey, 54 percent of those reached said they were not sure who they would vote for, and in the Gomez poll, 36 percent said they were still unsure.

Tonight's debate will be aired on WBZ and live-streamed on Boston.com, BostonGlobe.com, and cbsboston.com.

—Staff writer Matthew Q. Clarida will live-tweet tonight's proceedings, beginning at 7 p.m. Follow him on Twitter @MattClarida.