UPDATE, 5/19, 6:23a: Results:
Obama: 0-4 (has now lost endorsed races in MA, NJ, VA, PA)
AR Democrat US Senate primary: Incumbent Blanche Lincoln faces run-off against Bill Halter, since neither got 50% of the vote
KY Republican US Senate primary: Tea Party-supported Rand Paul beat establishment-supported Trey Grayson
PA Democrat US Senate primary: Joe Sestak defeated Obama-supported incumbent Arlen Specter
PA run-off to replace Congressman Jack Murtha: Democrat Mark Critz, Murtha’s former district director, trounced Republican Tim Burns…

5/18, 9:23a: There are likely more important races to watch today on Semi-Super Tuesday, but 4 I’ve got my eye on are in AR, KY, and PA.
Which primary and special election races are important and why? According to Family Research Council:

Both parties have haggled over the price tag of the new health care law, but the real cost may be in congressional seats. Voters have waited 2 months to tell Washington what they think of its “reforms,” and tomorrow, folks in AR, KY, OR, and PA will finally get their say. In a series of interesting primaries, at least 3 races have keen national significance. Two of them feature Democrats who made career-threatening decisions to back ObamaCare over their states’ objections.

In AR, Sen. Blanche Lincoln [pictured left] (D) isn’t getting any help from the Left-wing of her party in a campaign against Lt. Gov. Bill Halter (D) and D.C. Morrison (D) – a race so heated that it may take extra innings to decide. If that happens, Sen. Lincoln’s fate wouldn’t be decided until a June 8 run-off.
Her colleague in the Keystone State knows how it feels to be squeezed on all sides. Last year, in an act of desperate self-preservation, the Pennsylvanian fled to the Democratic Party.

What do you get when you cross an elephant and a donkey? Sen. Arlen Specter [pictured right] (D-Pa.) hopes the answer is “reelection.” Hoping to avoid a showdown with Republican Pat Toomey in November, Sen. Specter will be lucky just to get to the general election. He’s fighting for his political life against Rep. Joe Sestak (D), who could topple the once-Republican disappointment and end the senator’s 5-term reign.
Whether these leaders can survive anti-incumbency fever will provide some important clues as to how Congress will fare in its ultimate performance appraisal this November.

Meanwhile in KY, new blood is the name of the game. There, the tea party faithful are hoping to take another political scalp by beating the GOP’s Establishment pick, Trey Grayson (R) with Rep. Ron Paul’s (R-TX) son, Rand [pictured right].

There’s one more race I’m watching, as detailed by TalkingPointsMemo.com:

One of the top races to watch will be the special election for the Johnstown-area district formerly held by the late Rep. John Murtha, who passed away in February. The TPM Poll Average gives Republican businessman Tim Burns [pictured below right] an edge of 43.0%-42.4% over Democratic candidate and former Murtha aide Mark Critz.

And a final reminder from FRC to VOTE if your state is having a primary today!

Although voter turnout is traditionally low in primary season, polls are giving the enthusiasm edge to conservatives. Last week’s NBC/Wall Street Journal poll said Republicans had a commanding lead (65%) over Democrats (46%) in campaign interest. Can conservatives win on outrage alone? That’s for us to decide. Our leaders don’t get to Washington on their own. We put them there. If you’re disgusted about Capitol Hill’s war against life and liberty, go out there and do something about it! On Tuesday, those of you living in AR, KY, OR, and PA have an opportunity to exercise a right that people in the world are literally dying for.

Not only our soldiers but preborn babies will literally live or die depending on the outcome of some and the sum of these races.
[Top graphic via FRC]