Trump's claim is disingenuous in at least two ways:
1. Obama warned the public about Russia's meddling on October 7th, 2016. So, unless somehow Trump isn't following the election a month out, he was warned. This story got a bit burried by two other stories, those being hurricane Matthew and the release of the infamous Access Hollywood tape, the later seemingly timed to draw all the attention from the government's accusation against Russia.
https://www.washingtonpost.com/worl...0b9654-8cbf-11e6-875e-2c1bfe943b66_story.html
2. There is clear evidence on the record that Trump's campaign was an active participant with the Russians. That's why you're now hearing them try to say "collusion isn't a crimme". If somehow Trump had missed the news, he could have heard from his campaign manager, his son, his son-in-law, or any of the other people one degree of separation from him who were running his campaign and talking to the Russians about their efforts to help him win.
Trump can't plead ignorance.
They didn't "do nothing". They warned the public. They called Russia and told them that they knew what they were doing. But Obama was also under a lot of pressure not to be seen to be intervening in the election himself.
https://www.washingtonpost.com/graphics/2017/world/national-security/obama-putin-election-hacking/
Republicans actually pressured Obama not to push the issue too hard.
I'm not sure how it undermines Hillary's supposedly sure-fire victory to point out a criminal conspiracy to attack the election. It would actually play into Russia's narrative to undermine the credibility of the election. But the reality is, the election wasn't clean. It was badly compromised, and that's just the fact of the matter.
Lets try to stay on topic, shall we?