Not necessary, because whether the government is causally responsible for stock market growth has no impact on the reasoning. We know from a premise that voter confidence in the economy favors incumbent political leaders; whether those leaders or the government caused an improved economy doesn’t affect whether voter confidence can help keep those leaders in power.

Not necessary, because the argument doesn’t depend on a connection between investment choices and political choices. What matters is whether voters are confident in the economy. If so, we have reason to think incumbents are favored. This relationship holds regardless of whether people invest in a pattern that reflects their politics.

Necessary, because if this were not true — if the economic attitudes (such as confidence in economy) DO differ greatly from those of voters in general — then the fact investors are confident in the economy does NOT establish that voters are confident, too. And if the author can’t establish that voters are confident, he can’t establish that incumbents are currently favored.

Not necessary, because whether voters attribute the economy to previous governments has no effect on the reasoning. We know that voter confidence in the economy tends to favor incumbents. What matters is whether voters are currently confident in the economy. To whom they attribute the state of the economy has no bearing on whether voters are currently confident in the economy.

Not necessary, because the argument isn’t based on any claims related to “loyalty.” Even if voters are NOT usually more loyal to parties than to individual politicians, if voters are confident in the economy, we still have reason to think incumbents will be favored and therefore that the current party in power would likely retain power.