'Vote their conscience' or 'Vote with their conscience'

Question

Is the preposition required in this sentence?

Representatives should vote their conscience on Monday. OR Representatives should vote with their conscience on Monday.

Or are both okay? Is one better? Thanks

Answer 1

In U.S. English, "vote [one's] conscience" is a set phrase and is vastly more common than "vote with [one's] conscience." In fact, a Google Books search of the corpus American English for the period 1900–2008 for "vote your conscience" (blue line) versus "vote with your conscience" (too few instances to track), "vote my conscience" (red line) versus "vote with my conscience" (too few instances to track), "vote their conscience" (green line) versus "vote with their conscience" (too few instances to track), and "vote his conscience" (yellow line) versus "vote with his conscience" (too few instances to track) yields the following Ngram chart:

Searches for the various "vote with [one's] conscience" forms yield too few matches to register on the chart. It follows that, if the representatives are in a U.S. setting, it would be idiomatically normal to say that they "should vote their conscience on Monday," and it would be idiomatically odd to say that they "should vote with their conscience on Monday."

The situation changes somewhat when we shift from the corpus American English to all English language sources. Although the "with" versions of most of the expressions still don't garner enough matches to produce an Ngram line, "vote with their conscience" does. Here's the Ngram chart for "vote their conscience" (blue line) versus "vote with their conscience" (red line) for all English sources over the period 1900–2008:

Looking at the sources of the valid "with" versions (versions where vote is used as a verb), I found a fair number from the United States, but also significant numbers from Kenya, Canada, and the UK.

The earliest instance of "vote [one's] conscience" that a Google Books search finds is from John Fraser & ‎Charles Sergel, Sound Money (1895):

Clench is the worst and at the same time the best man they could have chosen for chairman. He represents the great mass of voters who go to the ballot box and vote their conscience.

And five years later, from Hezekiah Butterworth, In the Days of Jefferson: Or, The Six Golden Horseshoes (1900):

Reader, to preserve American institutions you must vote your conscience. In the new questions that will arise, may it not be well for you to ask, " What would Thomas Jefferson have done?"

He voted according to his conscience in every event, as all true men do, and as you must do if you would fulfill the ideals of the Declaration [of Independence].

And again from Hezekiah Butterworth, Traveller Tales of South Africa: Or Stories Which Picture Recent History (1900):

With nations as with men character is everything, as witness the conditions of the peaceful lands of northern Europe. The world would gain justice if men in elective communities would vote their conscience always. We must have the early American ideal to make a just world, and, until we have justice human blood will flow.

It thus appears that the idiomatic form that predominates in the United States today was already in some use at the turn of the twentieth century.

Answer 2

Both are fine.

"Vote your concience!", "Vote your district!" are catchphrases which became very popular when they were spoken to Democratic Representatives by then U.S. fictional party whip, Frank Underwood in House of Cards. Please visit the below link which is quite interesting.

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Vote your conscience is in the examples in Merriam-Webster.

4: to vote in accordance with or in the interest of "vote your conscience", "voted their pocketbooks"

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Both can be changed to "vote in accordance with your conscience / vote with your conscience" and "vote in the interest of your district".

Hope it helps.