Part political calculation, part self preservation, part naivety.
The political calculation was made the Republicans could not afford the loss of one Senate seat, and that Santorum needed Spectre on his side, not actively campaigning against him in '06. And Spectre would have done just that if removed by party rebellion. Whether he ran against Santorum himself or utilized his "friends" in Penn, he'd have waged war against Santorum.
I'm also of opinion Republicans can be too naive, or stupid if you prefer, and assume at the end of the day these people will be reasonable. From Democrats to Spectre this is a problem. Santorum probably did believe Spectre would feel an obligation to reward their support. Wrong as that thought is, I do think Reps entered into it with that mindset. It's a mindset that needs to be broken. Trust should be earned, not given under assumption the other person is reasonable.
I think Santorum made the wrong decision. I haven't argued against this. But do I think a failing once should define perception of that individual for life? No. He has a good record aside from this decision. He'd have to make other notable missteps to convince me this was a disturbing pattern rather than disappointing one time weakness under pressure. He did withstand high MSM pressure with his comments on marriage without backing down. Most Reps would have apologized and retracted under that pressure.
i dunno..i joined the republican party in 94 on the basis of the contract with america with republican assuring me that once lected they would change things..then over the years it became " well a bigger majority then we'll change things" and now they don't want to change anything at all in terms of government except to grow it and use it to establish their own power. Love me or hate me for saying but i'm really disillussioned about it when i spent 10 years watching for signs of a trend towards change and all i got was this lousy t-shirt