GOP Senators are opposing Obama's presumptive nominee for Secretary of Defense, Chuck Hagel. Their rationale is that he is weak on support for Israel and on sanctions against Iran.
That's plausible, but I'm not buying it. Republican senators are opposing Hagel's nomination for the same reason that they opposed Susan Rice's nomination for Secretary of State: they think they can take out the nominee, and thereby win a minor skirmish with Obama, and make him look weak. Susan Rice was a decent candidate for Secretary of State, but not great. There are generally 3 kinds of nominees for that job: career diplomats, prominent figures within the president's party, and people close to the president. Susan Rice is a career diplomat who is close to the president, but she's not a prominent figure within the party. As Republicans were opposing her, there were some other rumblings that she wasn't eminently qualified. So the GOP smelled blood in the water, and wanted a scalp. Fortunately for Obama, Rice figured out what was going on, and saved herself and the president a lot of grief.
They're doing the same thing with Hagel. This time there is some opposition to the nomination from the left, which isn't thrilled about the idea of another Democrat president choosing a Republican Secretary of Defense just to look tough on national security. So the GOP is betting that, if they gang up on Hagel, their opposition, combined with the qualms on the left, will sink the nomination, making Obama look weak on national security. They don't really care about whether or not Hagel is qualified or not to be Secretary of Defense; they just want to beat Obama somehow.