Hi!

So once the Emperor dies, what happens? Did all of the electors drop everything and rush to Aachen or wherever to vote in person?

Some clarifications first; Technically, the electors did not vote for the emperor directly; they instead voted for the position of King of the Romans, who shall succeed as Holy Roman Emperor once the current Emperor is removed from power naturally through death or a coup/abdication.

We could look at the primary document governing the Imperial elections, which is the Golden Bull of 1356 by Charles IV of Bohemia. [2]

To start, once the sitting emperor dies and the previous King of the Romans take the mantle of Emperor, or when the current King of the Romans dies while the Emperor still lives, all of the electors are called to participate in the elections by the Archbishop of Mainz. The electors are given three months to convene in the Free City of Frankfurt and start the election. All of this is detailed in art. 1 sec. 15

“ Moreover… [the] archbishop of Mainz at the time shall intimate this same election to the different princes, ecclesiastical and secular, his co-electors, by letters patent, through his envoys. ...And letters of this sort shall state that, within three successive months from the day expressed in the letters themselves, each and all of the prince electors ought to be settled at Frankfort on the Main, or to send their lawful envoys, at that time and to that place, with full and diverse power, and with their letters patent, signed with the great seal of each of them, to elect a king of the Romans and prospective emperor.” [2]

There are a lot of ceremonies detailed in the Golden Bull, like who should escort who, how would the Prince-Electors would conduct themselves going to, arriving in, and while staying in Frankfurt (art. 1 secs. 8-20,arts. 3, 4, 5 and 6), and how would the election process go (art. 2) Essentially, the electors go to a mass, then they would stay in Frankfurt to vote for 30 days until they have reached a simple majority. If the electors haven’t made a decision in those 30 days, then they are fed nothing but bread and water until they do. (art. 2 sec 4) [2]

However, these rules weren’t followed in some cases like in the election of Frederick III. He was a candidate for the position of King of the Romans when his uncle Albert II, the previous King of the Romans, died in 1439. Frederick III was voted in by 5 of the 6 electors by an extraordinary election in Nuremberg, not Frankfurt, with the Wittelsbach Louis IV Elector Palatine dissenting. This was so since Albert was only KotR for only a year and he was busy fighting off the Turks in the Balkans as King of Hungary, that he did not yet have the mandated first Imperial Diet which always happens in Nuremberg (as stated in art. 29 [2]) Thus, when the first official Imperial Diet under Albert II convened in Nuremberg in Feb 1440, he was actually already dead and the first agenda of this Diet was to elect a KotR. [4]

Also, yes, only 6 electors participated in that election since the throne of Bohemia was vacant (since unlike most in the HRE, the Czech nobility elected their own King which I discussed here and they haven’t voted in a new one, since the previous King of Bohemia was also Albert II) [4]

Could the electors just have someone vote in their place?

Yup, this is guaranteed under art. 1 secs. 8-20 [2]

What if the representative was bribed to misrepresent his elector?

The Constitution of the 1356 doesn’t have the specific law regarding this eventuality, but given that the Electors have free reign of justice over their subjects (and by extension, their representatives) due to the Privilegium de non Appellando they can decree that either their vote is deemed null or whatever they see fit and then hang the representative or something. I haven’t seen an outright event of this in any election post 1356, however, usually because the Electors themselves were able to attend most of the Imperial Elections even if they are deathly ill, as what Albrecht Achilles I of Brandenburg had done during the 1486 Imperial Elections. (He actually died after the Imperial Elections finished in Frankfurt)

Did electors declare their support for certain candidates before the emperor died or was it a secret until voting day?

Yes. An example of this was the explicit support of Frederick I Hohenzollern of Brandenburg for Frederick III of Austria, and another example is the significant disdain held by Frederick the Great against Maria Theresa and her husband Francis, which led to Brandenburg abstaining from voting in the 1745 Imperial Elections. [1]

What if one of the electors is on Crusade or ill or refuses to show up because he doesn't want a new emperor?

If the Elector himself is away, he gives someone, usually his regent, the authority to perform his duties at the Imperial Election.

If the elector refuses to attend after the mandated three months, then their vote is voided. Brandenburg was especially mentioned to this problem (which I think is due to their distance from Frankfurt) that it was given a specific article to confirm what happens to non-attending Electors in the Letters of Intimation, art. 18. [2]

What if the pope refuses to invest a new archbishop-elector after the old one dies in order to mess up the election?

A particular example of this was Gebhard von Waldburg who converted to Calvinism during his reign as Archbishop-Elector of Cologne in 1578. The complete loss of the Catholic majority of the Electors (which was made up of the three Electors Spiritual plus the King of Bohemia, which was held by the Catholic Habsburgs) created such a huge scandal that the Pope actually paid Spanish troops, amongst other Catholic forces, to depose von Waldburg and install a Catholic Archbishop-Elector [3] in the so-called Cologne War. [3]

Why did electors vote the way they did? Did the religious voters often accept the Pope's favorite for Emperor?

These are really wide questions that breaks rule 3 for me, but if I can opine my own opinion, political and religious exigencies are usually primary driving factors and that it greatly depends.

How long was the Empire willing to go without an Emperor?

The longest period that the HRE was without an undisputed emperor post-1356 was the interregnum between Charles IV and Sigismund (1378-1433).

Was the actual vote as unpredictable as election day in the US for example?

Given the fact that the Hapsburgs and their scion post 1440 won the election 16 times with only 1 interruption with the Wittelsbach emperor Charles VII? Usually not.

Sources:

[1] Mack Walker, Johann Jakob Moser and the Holy Roman Empire of the German Nation.

[2] The Golden Bull of 1356; Translated and Scanned by The Avalon Project of Yale University (Link: https://avalon.law.yale.edu/medieval/golden.asp)

[3] Hennes, Johann Heinrich. Der Kampf um das Erzstift Köln zur Zeit der Kurfürsten.

[4] Herre, Herman. (Editor) Deutsche Reichstagsakten unter Kaiser Friedrich III.: 1440-1441, Part 1