IN delivering a public reprimand to Sen. Alan Cranston (D) of California last week, the Senate Ethics Committee partly redeemed itself for its hesitant, kid-glove response to the Keating Five scandal.For all the squirming, weasel-wording, and partisan wrangling that characterized the committee's probe, the panel moved the marker higher in setting ethical standards for members of the Senate. It rebuked Senator Cranston even though his conduct didn't violate any specific law or rule of the Senate. Senators, the panel rightly indicated, will be held to generally understood, albeit unwritten, standards of ethical behavior. Cranston and his four colleagues (who earlier received far more lenient rebukes) - Democrats Donald Riegle of Michigan, Dennis DeConcini of Arizona, and John Glenn of Ohio, and Republican John McCain of Arizona - attempted in varying degrees to deflect federal thrift regulators from their scrutiny of Charles Keating and his failing Lincoln Savings & Loan. Over the period when the senators from four different states were performing what they called "constituent services" for Keating, each received large d onations from the thrift operator for their reelection coffers. Cranston received by far the largest amount from Keating - nearly $1 million - and under circumstances that appeared to be the most flagrantly improper. His claim last Wednesday that he was being unfairly singled out for actions that most other senators have taken was cynical and self-pitying. Still, temptations abound for all members of Congress. The campaign-finance system cries out for comprehensive reform. As an initial step, though, Congress should more tightly restrict the "soft money" loophole in the law that limits individual donations to lawmakers. Through the loophole, rich contributors pour millions of dollars into ostensibly independent election organizations actually controlled by candidates, such as those Cranston used to accept Keating's largess.