For performing such automation operations you might wanna use something that's built over selenium to handle all such tasks like tab switching , automatic element finding , Special Key press and hold :etc.
Take a look at : "webbot"
webbot works even for webpages with dynamically changing id and classnames and has more methods and features than selenium and mechanize.
Here's a small snippet
from webbot import Browser
web = Browser()
web.go_to('google.com')
web.click('Sign in')
web.type('mymail@gmail.com' , into='Email')
web.click('NEXT' , tag='span')
web.type('mypassword' , into='Password' , id='passwordFieldId') # specific selection
web.click('NEXT' , tag='span') # you are logged in ^_^
For your specific usage :
web.press( web.Key.CONTROL + 't' )