HBO's John Adams implies Hamilton and others rigged the first US Presidential Election to give Washington a landslide, in the interest of national unity. Were they basing this on anything historical?

We should be clear that Hamilton did not rig the election. The early Electoral College operated differently than today. At the time, each elector was bound to vote for 2 people, one of which could not be from their state. They didn't vote separately for President and Vice President. The second place vote receiver would become Vice President. Everyone understood that Washington was the choice for President and Adams for Vice President, but if all the electors voted for both Washington AND Adams, that would have left them tied. Then, the election would have been thrown to the House to resolve. This very thing would occur in the fiasco of 1800. Hamilton saw this danger and wanted to make sure that there was no confusion over Washington's election. He wanted to make sure that plenty of the other electors didn't pick Adams for their second vote, ensuring the election would be smooth.

It's true that Adams and Hamilton were not friendly and would become less so over the rest of Hamilton's life, but I haven't seen convincing evidence that Hamilton urged electors to avoid Adams due to any sort of vindictive motive.

I'm drawing from Wood's Empire of Liberty and Middlekauff's Glorious Cause.