Simply use ID Selector (“#id”) you do not need attribute selector with wild card if you have only one item to get. As ClientID
gives you complete and exact id of element.
alert($('#<%= ddlPortfolioName.ClientID %>').val());
If you are using framework 4 or above you can use Control.ClientIDMode to keep the Server id as ClientID
alert($('#ddlPortfolioName').val());
If you have dropdown in grid or repeater or listView then you will have to use contains wild card with attribute selector.
$('[id*=ddlPortfolioName]').each(function(){
alert($(this).val());
});