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Ronaldo has been the captain for awhile, so this should come as no surprise.
Unfortunately, he has picked up a slight reputation of disappearing in big games. He didn't have an impact in Euro 2008 or the latest World Cup. For Portugal, he has left much to be desired. For Real Madrid, it seemed like he was lost in every El Clasico.
Then, in the last match against Barcelona, he scored the winner. Ronaldo silenced those critics and proved that he could come up big when it matters. Of course, he followed that up by choking in penalties against Bayern Munich in the Champions League.
However, it must also be noted that he scored twice before that, including on a penalty. Making two penalties in a match is tough for any player. While Ronaldo should've converted it, it's reasonable to see how he didn't.
Ronaldo isn't known for being a fantastic leader, but he leads by example. If he plays well in this competition, Portugal could make some noise.
Obviously, don't expect him to do everything. Portugal has gone down that road before. If they keep games close and let Ronaldo take over when it matters most, Portugal has a chance of lifting the trophy.