The state's top elections official says fewer than one in five registered voters in Massachusetts could cast ballots in the U.S. Senate primaries.

Secretary of State William Galvin said Monday his ``best guess'' was for about 550,000 voters to take part in Tuesday's Democratic primary between U.S. Reps. Edward Markey and Stephen Lynch, down from the nearly 670,000 who cast Democratic ballots in the 2009 special primary following the death of Sen. Edward Kennedy.

Galvin notes that both Markey and Lynch have strong get-out-the-vote operations that could drive up turnout.

Galvin predicts about 200,000 people will cast votes in the Republican primary between Michael Sullivan, Gabriel Gomez and Daniel Winslow.

The winners of Tuesday's primaries will advance to the June 25 special election to fill the seat formerly held by John Kerry. [Associated Press]

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