How to make <button> fill a table cell <td>?

I have this HTML ...

<p align='center'>
<table width='100%'>
<tr>
<td align='center'><form><input type=submit value="click me"></form></td>
</tr>
</table>
</p>

... which results in a table and button that looks like this ...

... which is fine, but how would I make the button fill the entire cell?

Thanks!