While US congressmen are usually members of the Democratic or Republican party, they are theoretically under no obligation to vote according to the majority opinion of their party. But just how often do members of Congress or Senate vote against their own party?
According to the Brookings Institution, the answer for 2014 was
94% of House Democrats 95% of House Republicans 99% of Senate Democrats 90% of Senate Republicans
This is the percentage of members who voted with the majority of their party on average. Note that for any particular vote, this means that at least 50% have to vote with their party. Because whatever side more than 50% chooses is the one that counts.
If you want the against numbers, I guess subtract from 100%. But this is the form in which Brookings had them.
This data is from Table 4 in Chapter 8.
FiveThirtyEight has a more recent table for agreement with Donald Trump. However, they don't have an alternative way to measure how often Democrats agree with Democrats. And of course there may be some issues on which Congressional Republicans disagree with Trump while agreeing with each other. Also, this measure is on an individual basis rather than for the parties as a whole. I mention it because, well, at least it's current.