Does Mueller need more ammo to get to the top on any collusion type crimes?
Collusion is not a criminal offense. If he agreed to go light on Russia on sanctions or otherwise, either because they helped him win, because he owes them financially, or because they have dirt on him personally, the result SHOULD be that he is removed from office as it is effectively a form of treason.
But as others have pointed out that is largely a political question, and though I have no doubt that if proven to be the case 99 % of the Congress (including Republicans) would think it disqualifying, the fact is they are afraid of his 35 % and that they would buy into the claim that s just the establishment out to get him because they never liked him.
Obstruction, however, is a crime. The reason the tweet over the weekend was so significant is that it admitted that Trump knew that Flynn had lied to the FBI. Note that Trump -- who at one time took credit with the Russians for firing Comey as taking care of the Russia problem -- now claims that Comey is lying when he said that Trump pressured him to drop the investigation into Flynn.
So it comes down to who you believe, Comey, or Trump. Or even more particularly, do you believe Comey and 4 month-ago Trump, or just Trump this weekend. If:
1) Trump knew Flynn lied to the FBI;
and
2) Trump tried to get Comey to back off of investigating him,
then that is almost certainly obstruction.
And then of course you add on the firing of Comey, to boot.
I find it noteworthy that last week Sessions met privately with the House committee and was asked point blank whether Trump has ever pressured him to interfere and drop the Russia investigation.
Sessions refused to answer the question.