why should Congress be given a raise? What have the repugs and the Blue Dogs done to earn a raise?

http://www.infoplease.com/ipa/A0875856.html
http://www.fedsmith.com/2012/12/27/end-of-the-federal-pay-freeze-announced/

In 1989, Congress passed an amendment allowing for the automatic raises, unless lawmakers specifically voted to reject it. Which Congress did, until 2000.

http://www.legistorm.com/member_of_congress_salaries.html

In 1989 Congress voted to make pay raises automatic unless they voted to reject it.
http://usgovinfo.about.com/cs/agencies/a/raise4congress.htm

The Executive Order is to end the pay freeze on Federal Employees.
http://www.americanthinker.com/blog/2012/12/obama_signs_executive_order_giving_biden_congress_pay_raises.html
A side effect of this is that the Vice President and Congress will get a bump in pay. The real benefit will be for other govenment employees who have not seen a pay raise in three years.

could still vote to forego the raise?

under normal circumstances. Their automatic pay raise was somehow coupled with the the freeze on all federal workers.

so they would not have to be on record voting themselves a pay increase. They're sneaky that way.