How many decisions of the Supreme Court end up in a tight spot between judges considered to be "progressive" and those considered to be "conservative"? I know that 5-4 decisions are rare in the first place, but how many of them hinge on party lines in the court?
I manually recorded the votes on 2012 supreme court cases from the list on wikipedia. There's a page for each year since 2000 thats in the same format. You (or someone with some time on their hands here) could probably whip up a webscraping script to capture for each year.
For 2012, of the 20 5-4 split cases on the page, 14 were split along party lines with Kennedy swinging the vote. The remaining 6 cases were mixed. The supreme court took 79 cases in 2012, so cases split across ideological lines accounted for about 18% of cases in that year.
It is of course important to take into account that judicial philosophy doesn't completely correlate with political philosophy, which becomes more apparent when a justice strongly adheres to a judicial philosophy (as opposed to using a judicial philosophy to justify a politicized ruling). Several of those "mixed-party" votes were Scalia adhering to his Textualism which would lead to cases such as Maryland v. King, a fourth amendment case, in which Scalia was joined in his dissent by Ginsburg, Sotomayor, and Kagan.
So I looked up SCOTUS Blog which documents statics of the court opinions and decisions and found that this term (October 2017) was high with 23%, but there were often years very low. Last year, 5-4 decisions made up 10% of all cases, and the year previous to that it was 5%. Within the past 15 years, the October 06 term was the most divisive, with 1/3rd of the cases being 5-4 splits. Ideological splits (that is, Liberal verses conservatives) are the highest cause for the split court, but that doesn't mean all cases go to the conservatives. For example, of the four cases that were 5-4 in the Oct15 term, 3 were considered the liberal court victories to the conservative's one.
SCOTUSBlog lists one of three classifications for the 5-4 split Conservative (the four Conservative Justices plus Kennedy) Liberal (the four liberal justices plus Kennedy) and Other (Any combination of Conservative, Liberal, and Kennedy).
In the past 8 terms (Oct10 - Oct17) Liberals scored the highest percentage of 5-4 decisions in 3 out of 8 years. Conservatives scored the highest percentage in 4 out of 8. Other has never recieved the highest percentage of decisions. In Oct11 all three categories received an equal share of 5-4 decisions (33% of all decisions each). Oct17 (this past term) is the only term where liberals achieved no 5-4 decision victories. It was the best year of the sample for conservatives, where they had 73% of the decisions but the best year for any group of justices was Oct15, where Liberals took 75% of all 5-4s.
TL;DR: Most of the 5-4s of any year are ideologically motivated, but this split was only 100% for one year since Oct05 Court.