In Georgia, that's the last day for qualifying to run in the "General Primary", to be held on July 15.
While the average voter may not be aware of that date, it's a good bet that Democratic office-holders, like Georgia's John Lewis, are.
Lewis is a legend of the civil rights era: he was chairman of the Student Nonviolent Coordinating Committee (SNCC); he spoke at the March on Washignton in 1963, he marched in Selma in 1965, and is currently serving his 11th term as a Congressman from Georgia's 5th congressional district, which is comprised mainly the city of Atlanta. He is also a superdelegate who endorsed Hillary Clinton in October.
Georgia went for Obama by a margin of more than 2-to-1. Fulton County, which is essentially the city of Atlanta, went for Obama by a 3-to-1 margin. (Results for the 5th CD are not available; just by county.) Finally, exit polls indicate that African-American voters in Georgia supported Obama by close to 9-to-1 margins.
So should John Lewis be worried about a primary challenge? Probably not. After all, he's a icon of the Civil Rights movement, and has decades of good-will built up with the community. But he is probably not happy about having to explain a 'vote' that 75% to 90% of his constituents not only disagree with, but actually went to the polls and voted on.
Nor is he the only high-ranking African-American office-holder who has endorsed Hillary. Congresswoman Stephanie Tubbs (OH) endorsed in April; Sheila Jackson-Lee endorsed in May; and Maxine Waters endorsed just before the California primary.
It's considered 'bad form' for any incumbent Congressperson to face a primary challenge. Moreover, the challenger usually has trouble raising money, and faces the argument that the district will lose the seniority and clout in Congress that has been built up by the sitting Representative.
In 2000, Congressman Bobby Rush was completing his 4th term in Congress. Another leader in the Civil Rights Era (although a few years younger than Lewis), Rush had been a member of SNCC from 1966 to 1968 and was a co-founder of the Illinois Black Panther Party in 1968. He also was a Chicago Alderman for 8 years. (Rush himself had unseated an incumbent Congressman, Charles Hayes, in a Democratic primary in 1992.)
But in late 1999, Rush faced a challenge in the Democratic primary from a rising young star. Rush had the support of the party, including then-President Bill Clinton; in 1991, Rush had been the first Illinois elected official to support then-Governor Clinton. The challenger entered the race in September, 1999, six months before the primary, but campaigning was halted by all parties (including a third female candidate) in mid-October, when Mr. Rush's 29-year-old son was shot, and after four days in a Chicago hospital, died. As an observer noted, “That incident seemed to wash away any bad feelings that voters had or might have had about Bobby Rush.”
Bobby Rush survived that challenge in 2000, winning in the end by a comfortable 61-30% margin.
His opponent that winter? Then State Senator Barack Obama.
Footnote: Just last week, calling it "one of the most difficult decisions" he's made in politics, and after endorsing one of Obama's opponents in the 2004 Senate primary, Rush endorsed Obama for President. Said Rush:
We come from the same neighborhood and represent the same constituency, and I'm going to be with my constituency and Sen. Obama.