The down ticket races are where Sanders can really help. I don't think that does a whole lot for his leverage with Clinton, however. She doesn't strike me as someone who particularly cares whether she's negotiating right wing policies with a GOP congress or left wing policies with a Dem congress, just so long as she's in the Oval Office doing the negotiating - and there's essentially zero chance that she'll end up anywhere other than that.
Trump's main strategy at this point seems to be turning off as many people as possible about this election so that they stay home in disgust and his passionate base of supporters turn out to be enough to eke out a win. That strategy won't work well enough to get him elected, but it might work well enough to keep congress red. Sanders can effectively counter that possibility by getting his people out to the polls. He doesn't need to endorse her in order to do that, but it does make the job easier and if the party is still at odds, far more people will just grumble and stay home. He did enough to be able to make a massive difference in Dem policy and he can now work to keep that influence or grumble himself and let it slip away.
Trump's main strategy at this point seems to be turning off as many people as possible about this election so that they stay home in disgust and his passionate base of supporters turn out to be enough to eke out a win. That strategy won't work well enough to get him elected, but it might work well enough to keep congress red. Sanders can effectively counter that possibility by getting his people out to the polls. He doesn't need to endorse her in order to do that, but it does make the job easier and if the party is still at odds, far more people will just grumble and stay home. He did enough to be able to make a massive difference in Dem policy and he can now work to keep that influence or grumble himself and let it slip away.