Try making this changes to your code, Hope this helps!

{!REQUIRESCRIPT("/soap/ajax/26.0/connection.js")}

var url = parent.location.href;

var AccId = "{!Account.Id}";
var CampId ="{!Campaign.Id}";
var CampOwnerId= "{!Campaign.OwnerId}";

var updateRecords = [];

//Array to hold records to update

var updateRecords = new Array();
var update_Cons = new sforce.SObject("Contact");

var result = sforce.connection.query("SELECT Id, ContactId, LeadId, Contact.OwnerId, Lead.OwnerId, campaignid FROM CampaignMember WHERE campaignid = '"+CampId +"'"); 
var recs = result.getArray("records");

if(confirm("Are you sure you want to update the name on related contacts and leads?") )
{
for(var i = 0; i < recs.length; i++)
{

recs[i].Contact.OwnerId= CampOwnerId;
recs[i].Lead.OwnerId= CampOwnerId;

updateRecords.push(recs[i]);

//If the CampaignMember is a Contact, update the contact record

if(recs[i].ContactId != null){

    var contact = new sforce.SObject("Contact");

    contact .OwnerId= CampOwnerId;

    contact .Id = recs[i].ContactId;

    updateRecords.push(contact);

//If the CampaignMember is a Lead, update the leadrecord

}else if(recs[i].LeadId != null){

    var lead= new sforce.SObject("Lead");

    lead.OwnerId= CampOwnerId;

    lead.Id = recs[i].LeadId;

    updateRecords.push(lead);

}//End If
}

var result = sforce.connection.update(updateRecords);

parent.location.href = url;

if(result[0].success == 'true')
{
alert("Related contacts and leads updated successfully");
}
else
{
alert("Failed to update the related contacts and leads");
}

parent.location.href = url;
}