The Democrats are blowing out the Republicans in the special elections in 2017 and 2018. If the Democrats take the House and Senate, then impeach and convict Trump, it falls to Pence. However, Pence is mixed up with Trump's maneuverings and might well have committed impeachable offenses himself. If Trump gets impeached and convicted, then Pence gets impeached and convicted before he is able to nominate a VP to replace himself and get the nominee confirmed by the Senate, the presidency falls to the Speaker of the House. If the Democrats take the House, that Speaker will be Democratic.
Poor Nancy. The Republicans hate her, the Bernie Democrats hate her. She always seemed okay to me, frankly.
Oops, she's 77. Too old. She'll be okay until 2020, but she should not run for re-election. Most people, even quite sharp ones, admit that there is a little bit of a falloff after the early 70s. You're not quite at your peak. The presidency requires someone at the very peak of their mental powers.