A day before the Democratic Senate primary, Massachusetts Rep. Stephen Lynch has been sidelined by illness, his campaign said Monday morning.

"Due to illness, we have cancelled most public events today. However, tonight's rally in South Boston will take place as scheduled," Lynch spokesman Conor Yuntis said. Yuntis didn't provide any more details.

Lynch heads into Tuesday's election as the underdog against Rep. Ed Markey, who has the backing of the Democratic establishment. On the Republican side, former Navy SEAL Gabriel Gomez and former U.S. attorney Michael Sullivan are viewed as the two front-runners. The Democratic nominee will be a heavy favorite in the general election.