For a Republican to win a statewide race in Massachusetts, he has to attract voters from all parties.  But will Tuesday's endorsement of Republican gubernatorial candidate Charlie Baker by a Democratic mayor lead to more votes?

The South Shore is a battleground. In the 2010 election for governor, Baker won almost every local community. Quincy went to Democrat Deval Patrick, but the vote was split.  Former State Treasurer and Quincy resident Tim Cahill won 25 percent.

Now, Tom Koch, the Democratic mayor of Quincy, is crossing party lines to endorse Baker:

"There’s times you have to take a stand.  This is one of those times.  I believe the candidacy of Charlie Baker is perfectly clear.  He’s the right choice to lead the commonwealth going forward. Whether you’re democrat, republican or unenrolled," Koch said.

Koch supported Treasurer Steve Grossman in the Democratic primary.

The campaign of Democratic candidate and attorney general Martha Coakley says Koch is trying to get back at her for an ethics prosecution against Cahill, who is a Koch ally.  The Quincy mayor denies that played into his decision and he couldn't say whether more Grossman voters would follow his lead and support Baker.