The University of Massachusetts Lowell will host a debate featuring candidates for the US Senate seat representing Massachusetts, beginning at 6 p.m. Monday. The event will begin with any Republican candidates who choose to participate; businessman Gabriel Gomez, former US Attorney Michael Sullivan, and state Representative Dan Winslow are in the race and have been invited. At 7:30 p.m., US Representatives Stephen Lynch and Edward Markey, candidates in the Democratic primary, will take the stage. All candidates are vying for the seat left open by John Kerry, who resigned earlier this year to become the US secretary of state. Democratic and Republican primaries will be held April 30, with winners facing off in a special election June 25.