Que??"must have been close" to Putin's percentage? On what ground is your claim? That even the Western Wikipedia would not say. Although supported by Jelcin (similarly as Putin originally was) and in high political positions in 1990's his vote percentage was not enough to get into parliament. Changed later to Ukraine to help then president Yuschenko, it did not work either. Nor his involvement with oil tycoons and with infamous Brezinsky, you can learn a lot what all he had tried...
Even for a mayor of Sochi before Winter Olympic he ended up with 14%, whilst it was won with 77%, very close?
His only achievement was opposition to Putin and support by Western big people (e.g. McCain), but what else he could offer?
There goes a story (among many others) about his assassination: It was known to him and to Navalnyj (and warning either) - that within next days is something going on against them. In fact, not by Putin, but as a "reward" by Chechen people for their support of the huge demonstration for "j' suis Charlie". Navalnyi took it seriously, organized pamflet showcase and let himself got jailed securely for 2 weeks. Not the proud guy Nemtsov, showing off his beautiful Ukraine girlfriend model ...
The assassins were caught within 2 weeks - quite a different time frame unlike many other famous assassins...
Who was their sponsor who paid them 15 mil. rubles, it's not really known, but guessed...