My search of the coincidence of SCOTUS vacancies during an election year, limiting my search to the last 50 years, yielded up to 3 instances. There were prior instances--FDR had 2 SCOTUS nominations during 2 different election years, and Woodrow Wilson had 3 nominations during election years. But I want to focus on the post-WWII years. Since 1956, Eisenhower, Nixon, and Reagan have nominated a justice during (or just before) their election year--all three presidents were Republican and they had a Democrat Senate. Two were in their first term, and one was in his second term, about to be followed by his vice president.
The first was Eisenhower's nomination of William Brennan. Justice Minton retired on Oct 15, 1956, and Eisenhower acted quickly--he appointed Brennan by recess appointment the very next day, nominated him on Jan 14, 1957, and he was confirmed shortly thereafter. Eisenhower had been reelected by that point, in November, easily defeating the Democrat contender, Adlai Stevenson, by a 15% margin in the popular vote.
The second instance, only marginally relevant, was Nixon's appointments in late 1971. In September, 1971, two SCOTUS justices, Black and Harlan announced their retirements within days of each other, both for health reasons. Justice Black died shortly thereafter, and Harlan died in December. Nixon attempted to nominate several justices that suffered humiliating defeats. However, by mid-December, Justices Powell and Rehnquist had both been confirmed. The reason that Nixon's appointments aren't quite as relevant is that this all occurred the end of the year prior to the election, not during the election year itself. Nixon was reelected in November of 1972, wiping the floor with Democrat George McGovern, with almost a 25% margin of the popular vote.
The final instance was in 1987 when Reagan nominated Justice Kennedy. Again, this instance is only marginally relevant, since this occurred the year before the election. Justice Powell retired in 1987, and a very contentious confirmation process followed, where Bork was shot down, and Justice Kennedy, the current "swing vote" was nominated on Nov 30, and sworn in on February 1988.
This was Reagan's second to the last year in office, since he was nearing the end of his second term, with George H Bush, his Vice President, about to be reelected the following year. In that sense, this is the closest example to what might happen this year under Obama. First, it's most recent example, historically speaking. Second, both are in their second term (Nixon and Eisenhower were in their first terms). Third, as with the other examples, both Reagan and Obama face(d) a Senate of the opposite party--Reagan had a Democrat senate at this point, while Obama now has a Republican senate. However, all of this started in June of the previous year for Reagan when Powell announced his retirement, and Kennedy wasn't sworn in for another 8 months. Mitch McConnell, the current Senate Majority Leader, has already announced that Republicans will stonewall any attempts by Obama to get a new SCOTUS justice appointed.
There is a fourth instance, that might be closer than the other three, but in fundamental ways is different--Justice Warren announced his retirement in June of 1968, the year Nixon ran against Humphrey and Wallace. The retirement was to take effect when Johnson appointed Warren's successor. This process was stymied by Strom Thurmond in what is known colloquially as the "Thurmond Rule." Justice Burger was nominated in May of 1969, the year after Nixon won the election. I don't consider this a reasonable parallel to the situation of Scalia's death, in the sense that there was no vacant seat in Warren's case--he agreed to stay on until his replacement was found, so there was no national imperative. In the case of Scalia, the new term is about to begin, and the seat is empty. Further, Warren announced his retirement in June, while we are 4 months earlier in the cycle at this point.
