It’s the first big primary day of the 2010 midterm elections. The two races we’ll be most interested in are the two Democratic Senate primary contests in Pennsylvania and Arkansas.

In Arkansas, Lt. Gov. Bill Halter is challenging incumbent conservadem Blanche Lincoln. In Pennsylvania, Congressman Joe Sestak is running against Senator Arlen Specter. Tonight, we’ll hear the interview I did with Sestak on Friday, and we’ll also reach out to Will Bunch, senior writer at the Philadelphia Daily News to get his take on the contest as we wait for the polls to close.

My pal Howie Klein at has a few great primers on primary day: Happy Primary Day and Will Democratic Voters Dump Conservative Hacks Arlen and Blanche Tomorrow?

Another huge primary contest comes up on June 8 in California, when progressive challenger Marcy Winograd takes on incumbent Blue Dog Jane Harman. She’ll be with us at the beginning of the show (6pm ET) and will take listener questions at 800-795-1159.

We’ll also get an update on where the oil spewing from the floor of the Gulf of Mexico is headed from Dr. Jeff Masters, director of meteorology and co-founder of Weather UndergroundFollow Jeff’s blog here.