A question for McCain's supporters

The Bush tax cuts will automatically expire in 2010 unless congress votes to either make them permanent or at least extend them. John McCain's supporters tell us that his promise to extend the tax cuts is an important reason to vote for him (despite the fact that he voted against them).

I have a question for the McCain boosters. Since it is congress which must act to keep the tax cuts from sunsetting how, exactly, will McCain extend them?

I mean if the next congress was going to be dominated by Republicans McCain's promise to sign the extension which they would surely send him would be meaningful. But since the next congress is going to see the Democrat party increase its majority in both chambers and since the Democrats have promised their constituents that they will let the tax cuts expire, how is Mr. McCain going to force the Democrats in both the House and Senate to break their promise and extend tax cuts which they and the people who vote for them regard as unfairly aiding "the rich" and harming the poor?

I'd really like to know how John McCain will force the Democrat party to do something it doesn't want to do and has sworn not to do.