About quarter million (by delegating)
The plan is as follows: First (with much delight to your competitors) you must go to hell. Abyss to be specific. You can do that by using Plane Shift with the attuned tuning fork you have acquired beforehand.
There you will meet your eager volunteers: Nabassu (MToF p.135). They are of particular interest because of their ability:
Actions
Soul-Stealing Gaze. The nabassu targets one creature it can see within 30 feet of it. If the target can see the nabassu and isn't construct or undead, it must succeed on a DC 16 Charisma saving throw or reduce its hit point maximum by 13 (2d12) (...) The target dies if its hit point maximum is reduced to 0, and if the target is humanoid, it immediately rises as a ghoul under the nabassu's control
That means that a hard-working nabassu can convert a voter to your cause every 6 seconds with a 3/4 chance (DC16 vs +0 CHA and 4HP on commoner).
And the best thing about your new volunteers is that they should be eager to join your cause:
Demonic Infiltrators. Whenever magic pulls demons from Abyss to Material Plane, nabassus try to get summoned so that they can embark on a feast of souls there. If a nabassu is summoned, it tries to break free so that it can devour the soul of its summoner and then set out to feed on the souls of whatever creatures it can catch. One way a summoner can avoid this fate is by providing a steady supply of souls to the nabassu, which can cause the demon to be cooperative - for as long as the supply lasts.
But you wouldn't want your volunteers to be too eager and you need them to actually turn up for election, meaning you have to get them to enter a contract using Planar Binding. As your funds are limited only to 29 contractors + travel expenses you will need to focus on long term contracts\$^\dagger\$. That means you will be using your two 7th level spell slot as well as 8th or 9th level spell slots each day to cast recruit new volunteers. When there are only 10 days left to election you can start giving out shorter 10 day contracts using your two 6th level spell slots.
Not all of them will want to sign up though specifically it will only be 3/4 due to their +3 CHA bonus versus your DC19 spell save. (This factor as well as voter saves are added at the end)
As for getting the volunteers to the location of the potential voters you would use Teleportation Circle which for the 7 days it would be required would cost 350GP leaving 650GP of component budget mysteriously unaccounted for.
If all goes well and your volunteers work 24/7\$^{\dagger\dagger}\$ 7th level spell slot used for plane shift they can approach 14400 (24*60*60/6) voters per day. This gives us a following table:
| Days to election | Max voters approached by a nabassu until election | Max volunteers recruited |
|---|---|---|
| 14 | 201600 | 3\$^{\dagger\dagger\dagger}\$ |
| 13 | 187200 | 4 |
| 12 | 172800 | 4 |
| 11 | 158400 | 4 |
| 10 | 144000 | 6 |
| 9 | 129600 | 6 |
| 8 | 115200 | 2 |
In total that would ideally sum to 4550400 voters but as only 3/4 of volunteers would agree to contract that would mean only 3412800 voters approached. And as only 3/4 of voters would be successfully converted that would mean 2559600 votes. This doesn't account for some time lost each day to casting spells but I'm confident that the votes lost that way wouldn't amount to more than hundred thousand so that the result would be about quarter of a million.
If we use use the Wish spell to bypass the cost requirements of spells we can stretch our funds and bind 6 nabassu on day E(election)-8, 4 nabassu on E-7 and a single one on each day onward resulting in about 3215700 or third of a million
\$^\dagger\$ With unlimited budget you could also use 5th level spell slot each day to command a single nabassu to work for a day and return to you so you could cast 1 day planar binding on it again.
\$^{\dagger\dagger}\$ If the demons need to sleep 8h a day, the result would drop to 0.17 million
\$^{\dagger\dagger\dagger}\$ 7th level spell slot used for plane shift