Case study The nominations of Alito and Garland
Case study The nominations of Alito, Garland, Kavanaugh
When Justice Alito, a strong conservative, was nominated to replace Sandra Day O'Connor (a moderate swing justice) this led to strong opposition from the minority Democrat Party, with the late Senator Kennedy referring to it as a vote of a generation. The eventual appointment of Alito tipped the ideological bias on the court, firmly establishing a conservative majority. This can be easily illustrated with their change in attitude to campaign finance regulations and 1st amendment freedom of expression. In McConnell v FEC 2003 the majority delivered a 5-4 opinion, upholding the Bipartisan Campaign Reform Act 2002 and limiting campaign expenditure in US elections. After Alito replaced O'Connor, the court reversed this decision in Citizens United v FEC 2010, undermining the regulations.
The death of conservative Justice Scalia gave Obama the chance to replace a conservative with a liberal, Merrick Garland and to tip the ideological balance on the court from a 5-4 conservative majority to a 5-4 liberal one. The subsequent refusal of Senate majority Republicans to hold confirmation hearings, thus blocking the nomination until President Trump was in office, is evidence of both the importance of this particular nomination as well as the polarised nature of the process. Trump's nomination of Gorsuch has less significance in changing the composition and outlook of the court (compared to its composition with Scalia in office) than Obama's would have done.
In 2018 Anthony Kennedy announced his retirement and Trump nominated Bret Kavanaugh - this is far more significant sine Kenndy is seen as a 'swing' judge who was liberal on social issues and conservative on fiscal (money) issues. Kavanaugh has a record of conservative views and opposition to abortion.