Is there a way to have a base class for a razor's code behind?

I have CampaignPage.razor:

@page "/Manager/Campaign"
@implements IDisposable

<PageTitle>Campaign</PageTitle>

<EditForm EditContext="EditContext"
          OnValidSubmit="HandleValidSubmitAsync"
          OnInvalidSubmit="HandleInvalidSubmitAsync"
          Context="EditFormContext">
...

And I have CampaignPage.razor.cs:

    public partial class CampaignPage
    {
...

Is there some way I can have:

    public partial class CampaignPage : OrganizationPage
    {
...

I tried doing this as:

public class OrganizationPage : ComponentBase
{
...

but that got me compiler errors.

You are almost there, since you are using partial class for CampaignPage so you also need to inherit you CampaignPage.razor from OrganizationPage

Your CampaignPage.razor should look like this

@page "/Manager/Campaign"
@implements IDisposable    
@inherits OrganizationPage