Why didn't the Democratic party make the smart move and make Mike Gravel their nominee

When Mike Gravel lost the democratic nomination he left the democratic party and ran as a libertarian. Libertarian ideology is very dissimilar to democratic ideology. Gravel has a great and lengthy resume, but did not propose a great plan. He had not won a single election in the last quarter of a century as he was unable to relate to voters. I believe Mike Gravel is a great and moral human being, but I do not believe he was the best candidate for the job. Further, he would have been extremely unlikely to beat John McCain had he won the democratic nomination. His record and background are very similar to McCain's, and McCain has more relevance given his current position of being a senator for the entire previous quarter century and his record of having been a moderate giving him the vast majority of independents.